How Do Compromise Rail Joints Connect Different Rails?

Why Different Rails Need to Meet

Track is not always built from one rail section. During line upgrades, a new heavier rail is often laid alongside an existing lighter one; at national borders or standard-change points, one rail standard ends and another begins; and at the approach to a bridge or a turnout, a different section may be needed. Where two rails of different profile meet, a plain fishplated joint cannot work - the joint bars would not fit both profiles and the running surface would step. The connection is made with a compromise rail joint.

What a Compromise Joint Looks Like

A compromise joint uses a pair of special fishplates (transitional joint bars) machined so that one half matches the profile of the first rail and the other half matches the second. Because the two rails have different head widths and web positions, the fishplate is asymmetric: the bolt holes, the bearing surfaces and the outer faces are all positioned to suit each rail. In practice, the joint is usually made with the two rail ends brought together and the transitional bars bolted through the webs, with the bolt spacing arranged so that each rail end receives at least two bolts in its own section.

The Critical Detail: Running Surface Alignment

The purpose of the joint is not simply to hold the rails together - it is to carry the wheel across the change of section without impact. That requires the running surface (the top of the head) of the two rails to be aligned, and the gauge corners to match as closely as possible. Because the two sections have different total heights, the joint is normally made on a raised or lowered base or with special shims so that the head heights are equal at the joint. A misaligned compromise joint is worse than no joint: the step in the running surface generates impact loads that can batter the rail ends and the joint bars. This is why compromise joints are machined to close tolerances and inspected regularly.

Typical Locations for Compromise Joints

Transition between a lighter section and a heavier section during phased track renewal.

Connection between UIC and other standard families (for example UIC54 to UIC60, or between Chinese 50 kg/m and 60 kg/m sections).

Approaches to bridges, tunnels and turnouts where the design requires a different section.

Mining and industrial networks where different suppliers' rails are joined on site.

Buying a Compromise Joint

To supply the correct joint, the manufacturer needs the exact profiles of both rails - the section standard, the nominal weight and, ideally, the profile drawing or rail sample. The bolt holes on the two ends differ in pitch and possibly in diameter, and the fishplate material must match the strength of the weaker rail. When ordering, state both rail sections, the rail height difference, and whether the track is jointed or continuously welded, because the drilling pattern must suit the existing rail ends.

FAQ

Can I weld a compromise joint instead of bolting it?

In many renewals the transitional connection is made by welding the two different sections with a special weld procedure, but that is a permanent works operation requiring qualified welders and testing. The bolted compromise fishplate is the classic, reversible solution and is still standard for temporary or phased connections.

Do compromise joints need more maintenance than normal joints?

Yes. The asymmetric geometry and the profile change make them more sensitive to impact, so bolt tightening, lubrication and measurement of the running surface step should be on the regular inspection list.

What is the maximum section difference a compromise joint can handle?

There is no hard limit, but the practical rule is that the two rails should be reasonably close in height and web position. Joining a light rail to a heavy mainline rail is possible with custom machining, but the joint becomes a permanent weak point and a weld or a transition rail is often preferred.

Can a compromise joint be used between rails of different standards?

Yes - that is one of its main uses. The joint bars are custom machined to fit each standard's profile, so a UIC rail can be joined to an AREMA or GB rail section at a transition point.

How do I specify the right compromise fishplate?

Provide the section designations of both rails, the height difference, the bolt hole requirements and the rail steel grade. The supplier machines the bars to fit both profiles, and the bolt holes on each half are drilled to match the corresponding rail.