Movable bridges are relatively common in areas where railways must cross navigation channels. The rails must be made discontinuous where the movable and fixed portions of the bridge deck interface. It is not possible to provide a simple rail cut at these locations because a means of carrying the wheels over the gap and allowing for bridge structure expansion must be provided. It is also necessary to ensure that the rails align properly with each other horizontally and vertically.

One of the early methods of getting around the rail gap problem was to cut or machine the rail ends at a 45-degree angle. This "mitre cut" assured that some portion of the wheel tread would remain supported by either one or both of the contiguous rails, and permitted a rail gap large enough for practicable movable bridge operations, whether dealing with a vertical lift, bascule, or swing span structure.

PB's transit and rail specialists have observed and evaluated various mitre rail designs during the course of our project work. The basic introduction to mitre rails provided below can be used as a general guide to mitre rail use from a track work engineering perspective.

Mitre Rail Basics

The Assembly. The mitre rail consists of a fixed point and a movable point that are mated in a bed plate or casting to support the rail and allow the passage of railway engines and cars. A mitre rail is required for each rail on each end of the movable span.44 An entire mitre rail assembly consists of the following general components:

• Mitred rail points
• Approach rails
• Lift plates or bed plates
• Rail fasteners
• Lifting rods and connections (for swing bridges)
• Tie pads
• Rail joints
• Rail locks
• Electrical insulation
• Signaling point detectors
• Mechanical and electrical systems that make all of the moving parts of the overall assembly function.

In the interests of keeping this article to a reasonable length, this discussion is limited to the mitre rail from the top of the rail to the top of the bridge tie.

The Mechanics. Bridge openings are created by either lifting the bridge mechanically or swinging the movable span on a horizontal plane. To accomplish a lift, as would be required for a vertical lift or bascule bridge, the movable point of the mitre rail is unlocked mechanically and lifted perpendicular to the line of the track with the span. On a swing bridge, the movable point of the mitre rail is mechanically hinged to permit the point end to be lifted vertically and, thus, clear the fixed point of the mitre rail at the approach span. Once the lift rail has been raised, the bridge span can be swung on a horizontal plane to open the navigation channel.

Tracks on most movable railway bridges are of an "open deck" configuration where the rails are fastened to timber bridge ties. The ties are connected structurally to the girders or stringers rather than being embedded in ballast as in ordinary track, since ballast cannot reliably hold the track to rigid alignment tolerances when the bridge deck is frequently moved. Moreover, a ballasted deck would obviously be impossible on a bascule bridge!

As such, mitre rails are usually mounted directly on bridge timbers. In some installations, however, steel ties are substituted for timber underneath the mitre rails. When this is the case, elastomeric pads similar to the direct fixation rail fasteners used commonly in rail transit tracks are often placed between the mitre rails and the steel ties so as to provide
some resiliency and electrical isolation.

Operating Speeds. The maximum permissible train operating speeds over a mitre rail will vary depending on the design and condition of both the mitre rail and the bridge superstructure. Normally, the maximum allowable speed is 72 kph (45 mph), although some mitre rail designs permit 97 kph (60 mph) for passenger train operation. During our investigations, we learned that mitre rails on higher speed rail systems are American phenomena, however. European railways simply do not use movable bridges on high-speed routes. Instead, they make the infrastructure investment to build fixed bridges that provide adequate clearance over the navigation channel.

Mitre Rail Design

The design of a mitre rail is that of a close tolerance assembly that must be capable of handling large magnitude and high impact railroad wheel forces in an exposed environment. The requirement of the rail to move creates difficulty in achieving rigidity in the track structure, and the mating rail surfaces must be able to compensate for a certain amount of dirt and misalignment if they are to be reliable.

Evolving from the very crude designs of the late nineteenth century, mitre rails became far more robust and sophisticated by the early twentieth century as it became essential to handle increased wheel loads and higher operating speeds. Six general mitre rail design types currently in use on U.S. freight and passenger railways are:

• Blunt nose type
• Fabricated rider rail
• Cast manganese two-piece
• Cast manganese three-piece
• Switch point type
• Sliding block type.

There is very little published information on the design, inspection and maintenance of mitre rails. Most of what is available consists of general criteria on inspection and maintenance. Fortunately, we have been able to obtain sufficient information from operating railroads and track work manufacturers to make a reasonable comparative review of the various mitre rail designs, as follows.

Figure 1: PRR Style Mitre Rail

Blunt Nose Type

The blunt nose mitre rail in various configurations is the oldest and simplest of the mitre rail designs in railway service. Both the movable and fixed mitre rail points are fabricated from either rail steel or manganese steel rail sections. Other terms used to describe this type of mitre rail include "overlapped" and "PRR type." The latter description originated in the use of the blunt nose mitre rail as an early track standard of the former Pennsylvania Rail Road.

Limitations and Disadvantages. In general, the blunt nose mitre rail is not used for operating speeds significantly greater than 40 kph (25 mph). The short wheel transfer distance in this design cannot accommodate all combinations of new and worn vehicle wheels optimally, and the wheel transfer over the blunt nose mitre rail tends to be noisy, even under the best installation conditions.

There also appears to be a limit to the rail expansion distance that the blunt nose mitre rail design can handle-approximately 76 mm (3 inches). This restriction is caused by the gap at the mitre rail point tips and by the maximum practical clearances between the guide block and alignment cradle typically used in this design (Figure 1). Lengthening these components is feasible technically, but doing so adds significantly to the complexity of manufacture and cost, thereby defeating the primary advantages of this design.

Rail steel at the running surface tends to exhibit plastic flow under repeated loadings. Our interviews with railroad maintenance personnel indicate that a continual grinding program is necessary to remove rail flow from blunt nose mitre rail ends. To some degree, this problem affects all mitre rail designs.

Applications and Advantages. Despite the higher noise and routine maintenance requirements of the blunt nose mitre rail compared with other configurations, it has some applications where lighter wheel loads predominate, such as transit and dedicated commuter rail operations. The blunt nose mitre is relatively simple and inexpensive. Replacement mitre rail points can be fabricated by virtually any track work manufacturer, or even a competent machine shop with no other track work experience, if necessary.

This mitre rail design appears to function acceptably in low speed, low tonnage track as long as the dynamic wheel loading (pounding) at the mitre joint can be tolerated by the underlying structure. Examples of it can be observed on the Amtrak/New Jersey Transit bridge immediately north of Newark New Jersey's Penn Station and on the Chicago Transit Authority's Ravenswood Line bridge across the Chicago River. Although a complete inventory of railway mitre rails outside of the U.S. has not been documented fully, a form of blunt nose design appears to be the predominant mitre rail used in service outside of the U.S.

It should be noted that an elastomeric pad under the bed plates is not considered to be effective for the control of the wheel pounding typical of blunt nose mitre rails. A thicker pad with a greater deflection has been observed to result in rail profile maintenance problems and appears to create unacceptable fatigue stresses on the bed plates, rail assembly bolts, and anchor screws/bolts.

Figure 2: Fabricated Two-piece Rider-Type Mitre Rail

Fabricated Rider Rail

This type of mitre rail is appropriate for bascule and lift bridges. One of the original designs dates from a 1944 installation on the former New York New Haven & Hartford Railroad. Although the early versions of these assemblies were replaced eventually with Conley two-piece mitre rails, the concept has been refined for use in modern heavy-haul railroad operations.

CMI/Promex manufactures an updated mitre rail design (Ridex^®) that utilizes a rider rail, also known as a carrier rail, easer rail or wheel riser. Rider rail designs are found primarily in the northeastern U.S., although the rider rail installation with the heaviest annual gross tonnage is located on the former Conrail bridge across the Maumee River in Toledo, Ohio.

Figure 2 shows the general arrangement of the fabricated rider rail. A square cut mitre rail is left with a sufficient gap to cover maximum running rail expansion and sufficient space for the rail to open and close. This usually requires a 51-mm to 76-mm (2-inch to 3-inch) opening in the running rail at the mitre joint. The rider rail is mounted on the outside of the running rail, either level with or slightly above its elevation. The outside portion of the wheel tread rides on the rider rail as it passes, and the wheel is thus carried over the rail joint.

The rider rail design has been updated to eliminate many of the original bolted rail connections and to utilize improved rail components, such as 500 BHN high strength steel.

Advantages. Interviews with both the manufacturer and operating railroads indicate that the life expectancy of the Ridex^® mitre rail design is approximately 300 MGT (273 MGTn)-approximately twice that of the manganese casting mitre rail designs described below-before major refurbishment or replacement is required. Primary wear on the assembly is the rider rail itself, which can be replaced easily with four bolts. The initial cost of the Ridex^® mitre rail assembly is much higher than other types, however.

Restrictions. The primary physical restriction to the Ridex^® and all other fabricated rider rail designs is that the length of the rider rail is relatively short. Because the rider rail acts as a significant wheel riser for worn and hollow wheel profiles, the operating speed over this type of mitre rail has been limited generally to speeds below 73 kph (45 mph). The rider rails on the mitre rail cannot be lengthened easily to permit higher speeds.

The most recent Ridex^® installations have reduced the number of bolts significantly by utilizing Pandrol rail clips for attachment to base plates. The Ridex^® mitre rail has a number of other attractive features, primarily in terms of durability and maintenance for use in higher tonnage freight traffic. Operation above 73 kph (45 mph) may require longer rider rails and more extensive rail support, adding a major cost to what is already a comparatively expensive product. Also, bridge designs requiring extreme rail expansion tend to cause a poor transfer of hollow wheels to the rider rail on the movable bridge side of the mitre rail. However, the main disadvantage to the premium Ridex^® version of the fabricated mitre rail appears to be its initial cost, not its service record.

Figure 3: Conley two-piece mitre rail for bascule and vertical lift bridges

Cast Manganese (Conley) Two Piece

This mitre rail is the most common type used on bascule and vertical lift bridges in the U.S., and the term "Conley Joint" has become a generic label for a cast manganese mitre rail. The design patent for this particular design of cast manganese mitre rail was granted to Conley Frog/Switch & Forge Co. in 1952. Blanchard Steel, a subsidiary of Cleveland Track Materials, began casting a similar design when Conley's patents expired in the mid-1990s, and is the only other U.S. manufacturer of this type of mitre rail. The following comments are based on Conley mitre rail designs and installations because the Blanchard Steel mitre rail has an extremely short service record, but they are generally applicable to all cast manganese mitre rails.

The Conley two-piece mitre rail consists of two large manganese steel castings (Figure 3). The fixed-point side of the standard assembly is a 2.134-m (7-foot) casting with the base plate, mitre point, and heel arm incorporated into a single unit. The base plate at the fixed mitre rail point is approximately 0.508 m (20 inches) wide to provide a base for the movable mitre rail point and guardrail. Unlike fabricated assemblies, the guardrail is an integral part of the mitre rail casting. The movable point casting includes the base plate, point, heel arm, and a short guardrail in a single casting. The opening between the mitre rail and guardrail forms a rough V-notch cradle that fits around the fixed mitre rail point, locating it laterally.

The mitre rail gap is at a low transverse angle to permit a relatively smooth transfer across the rail gap. The base plate castings are installed with a nominal 57-mm (2 1/4-inch) gap at 16o C (60o F), and can handle a relatively large rail gap in extreme cases-as much as 178 mm (7 inches). Explosion hardening is used at the running surfaces of the casting to initially reduce the metal flow typical of manganese steel in railroad service.

Advantages. The wheel transfer across this mitre rail is relatively smooth if the rail profile surface and adequate rail support are maintained. This design has a much longer transfer zone than the typical blunt nose mitre rail assembly. Relatively high operating speeds (up to 97 kph (60 mph) have been run across the Conley two-piece mitre rails, but required maintenance and inspection levels appear to increase dramatically for higher annual tonnage and operating speeds.

Limitations. One inherent weakness in the Conley two-piece mitre rail design is the proximity of the mitre rail gap to two adjacent bolted rail joints. If the heel bolts become loose or the base plate/tie support under the castings deteriorate, the mitre rail assembly can become extremely noisy, and wear on the mitre rail points or guardrail can occur very quickly. Moreover, guardrail used to keep the wheel flange from contacting the mitre rail joint also tends to exhibit rapid flange-way widening due to wear if good track line and surface are not maintained.

In revenue service, a certain amount of metal flow, particularly at the gauge corners, can be expected. As with the blunt nose mitre rail design, the most important routine maintenance item is the use of periodic grinding of the mitre rail casting.

Under the current maintenance grinding and welding practices on the various freight railroads contacted for the study, it appears that the Conley mitre rail assembly has an effective life of about 136 MGTn (150 MGT) before the entire unit must be replaced. Where cast manganese castings are changed frequently, one problem is matching the new mitre rail casting with the worn approach running rail profile. The cost of the Conley mitre rail casting is comparatively low so some inconvenience is tolerable.

In locations where the large casting footprint is not critical and where annual rail tonnage is in the low-to-medium range, such as 14 MGTn (15 MGT) or less, Conley two-piece mitre rail seems to retain a definite overall advantage over competing designs. Most movable bridge locations tend to have low rail traffic volumes, which explains the overall popularity of this design.

Figure 4: Conley Three-piece Mitre Rail

Cast Manganese (Conley) Three-Piece

The same general comments that apply to the cast manganese (Conley) two-piece mitre rail can be repeated for the three-piece version that is used for swing bridge spans (Figure 4). The major difference between the two types
is that the three-piece essentially has two separate mitre rail joints back-to-back with a hinge pin installed through one of the mitre rail joints. This design results in a short mitre rail assembly (rocker rail) that can be pivoted about the hinge pin, providing the clearance necessary to permit the bridge span to swing. Figure 5 shows the Conley three-piece mitre rail in open and closed positions.

Disadvantages. The hinged mitre rail joint results in a large number of rail joints in a 5.029-m (16.5-foot) length. All disadvantages that apply to the Conley two-piece mitre rail are characteristic of the Conley three-piece version, except that the three-piece assembly is even less tolerant of surface and line deficiencies.

Rapid wear on the flange way guardrail is a common feature of the cast manganese three-piece design, leading to deterioration of the mitre rail joint over time on the opposing running rail gauge corner. Also, the hinge and mitre rail joints are approximately the same distance apart as a typical inter-city railroad passenger car truck's axle spacing, creating increased batter on both joints.

Figure 5: Conley Three-piece Mitre Rail For Swing Bridges (Top) Rails In Lifted Position, Ready for Span Opening (Bottom) Rails In Closed Position, Ready For Operation

Maximum operating speed over bridges with this type of mitre rail is typically restricted to 97 kph (60 mph), even under optimal conditions. As noted for the Conley two-piece mitre rail, maintenance requirements under such loadings are quite high. Fortunately for track work maintenance personnel, the bridge structure itself tends to restrict operating speeds to a more reasonable level.

The cast manganese mitre rail configuration is particularly vulnerable to deterioration in rail surface, given its combination of solid castings and multiple rail discontinuities. With this in mind, note that Conley three-piece mitre rail assemblies on steel ties with soft elastomeric pads have deteriorated faster than when mounted on conventional wood bridge timber.

Advantages. For both the two- and three-piece designs, the use of a base plate under the cast-manganese mitre rail casting seems to have relatively few drawbacks. At the same time, they permit increased standardization of the assembly by allowing variations in tie spacing on the bridge structure.

The Conley three-piece mitre rail is the most commonly used configuration on swing bridges, despite some of the disadvantages noted above.

Switch Point Type

A premium mitre rail design that features a relatively long wheel transfer distance was developed by Amtrak and manufactured by Promex in the early 1990s. This design was intended originally for speeds as high as 145 kph (90 mph) and for a long service life under severe traffic loads, which would befit the high cost of the initial mitre rail installation. It has been installed on only two Amtrak bridges so far; one in New Jersey and one at the north tip of Manhattan island. The unexpectedly high degree of inspection and maintenance it requires, however, coupled with actual in-service operating problems, has made it unlikely that this specific design will be used on other movable bridges.

One of the main problems is the loosening (tension loss) of the many bolted connections on the mitre rail assembly. The bolting of the side bar for one mitre rail (lift and approach rail) alone required 21 bolts. Figure 6 shows that the base plates are bolted to the steel dual block ties, which are then bolted to the bridge girders. Also, this design is dependent on the durability of the elastomer and the maintenance of line and surface.

Figure 6: Promex Type Mitre Rail

Sliding Block Type

The sliding block type mitre rail (Figure 7) is a variation on the rider rail design that was developed by the former New York Central Railroad. It uses a large casting to slip around the base and sides of the rail when the bridge span is closed. The casting acts both as a means of keeping the rail ends in proper alignment and also provides a rider rail to support wheels passing over the joint gap. While it can be used on any type of movable bridge, its logical application is for a horizontal swing bridge. Examples can be seen at Amtrak's Renssalear Bridge in New York State and there are two examples near Chicago.

Advantages. The sliding block design has a few advantages over other systems that should be noted:

• The sliding block does not require a lift rail assembly for its operation.
• Because the sliding block design covers the rail gap in its operation, it is not affected by snow as much as the other mitre rail designs.

Disadvantages. The sliding block design has a number of drawbacks that have limited its its use:

• The sliding block does not operate well with even a small vertical misalignment of the rail base plates during the bridge closing. It is highly dependent on the accuracy and level of maintenance of the bridge movement mechanisms.
• Any wear in the casting at the rail base creates a misalignment in the rail gap. This misalignment could be made worse by the passing of new wheels.
• The additional operating mechanisms to slide the blocks back and forth add considerable initial cost to the overall assembly when applied to a bascule or vertical lift bridge. (The initial mitre rail assembly costs are comparable to those of the Conley three-piece design for a swing bridge.)
• Heavy longitudinal impact due to traffic (pulling and pushing ) forces.

Figure 7: Sliding Block Type Mitre Rail

Because there are existing installations still in use, it would be incorrect to state that the design was completely inadequate, but a new assembly would require a unique fabrication and provide essentially the same functional service as the Ridex^® mitre rail. Replacement of this type of mitre rail, if encountered, would require consideration of a different type of mitre rail design rather than replacement-in-kind.

General Installation Considerations

Although the primary intent of this article is to describe the major types of existing mitre rail designs for movable bridges, a few words should be devoted to the actual installation of the mitre rails.

Protection Against CWR Expansion. In most movable bridge designs, the rail on the bridge span has bolted rail joints, is selectively anchored longitudinally to act in concert with the bridge structure span, or is otherwise controlled against unwanted excessive thermal expansion and contraction. It is often the case, however, that the running rails leading to each bridge approach on the "land" (fixed) side are continuous welded rail (CWR). As the mitre rail is a rail discontinuity, even well anchored CWR can and will expand and contract with significant force on the fixed rail side of the mitre rail assembly. Control of this type of rail movement is normally counteracted by separate rail expansion joints. This is an especially important consideration when estimating the cost of new installations.

Mitre Rail Surface and Tie Pad Resiliency. A certain amount of impact force (wheel pounding), which is inherent in all mitre rail designs because of the necessity of wheel load transfers over a rail discontinuity, must be accommodated in the track and bridge structure design. Traditionally, this impact loading was absorbed by the wood bridge ties on which mitre rail assemblies were mounted. Beginning in the 1970s, however, a number of designs included elastomer sandwich type resilient tie pads on wood ties to reduce shock loading on the bridge.

Resilient tie pads in some form are required on steel ties. However, there is a tendency to consider excessively soft tie pads, which often results in excessive rail surface deflection. This deflection has a tendency to worsen the rail transfer across the mitre rail and create premature deterioration of both the tie pad elastomer and the mitre rail itself on heavily used bridges. In some cases, particularly on track with lower traffic volumes, additional pad resilience is used to compensate for a limited amount of rail misalignment.

Mitre rail installations on steel ties or steel joists should be designed conservatively with a minimum of bolted connections in both the mitre rail joints and in the rail anchoring to the bridge deck, and with particular attention to the base plate material properties.

Mitre Rail Grinding and Maintenance. A final observation that pertains primarily to track maintenance bears mentioning, particularly for cast manganese mitre rail installations. Frequent grinding and welding are required to prevent propagation of surface cracks and metal flow. Such maintenance, however, often leads to premature mitre rail deterioration and loss of ride quality. Mitre rail manufacturers generally include a maintenance manual that covers the care and feeding of these critical assemblies, and it is essential that a copy be placed in the hands of the client's maintenance staff at the end of the project.