Rail chain signal

A rail chain signal is a rail signal variant used to control train movement through junctions and other shared rail segments. Like a regular signal, it divides rails into blocks and prevents a train from entering the next block if that block is obstructed. The important difference is that a chain signal also looks ahead to the exit signals of the next block and only allows entry if the train can continue through the section according to its path.

Chain signals are especially useful for crossings, merges, and other places where a train should not stop in the middle of a shared section. They are placed on the right side of the track like normal rail signals. If trains need to run in both directions on the same track segment, signals must be placed on both sides of the rail, opposite each other.

A chain signal behaves according to the exits beyond the block it guards:

• If all exits are free, it turns green.
• If a train has reserved a path through the block, it turns yellow, and the other entrance signals for that block turn red.
• If all exits are occupied, it turns red.
• If some exits are free but not all, it turns blue; in this case whether a train stops depends on its own route.
• If it is blinking, it is not on rail or it cannot divide the rail into separate blocks.

A special case applies when the train’s destination can be reached from the chain signal without crossing any other signals. In that case, the chain signal does not consider the exit signals, and this applies to both train stops and temporary stops. A chain signal with only one possible exit will not allow a train into the block if the train would be forced to stop there.

This makes chain signals the preferred tool for preventing deadlocks and keeping intersections flowing. A train waiting at a chain signal will stop before entering a crossing if it cannot leave the shared section immediately, which prevents it from blocking other routes. On a crossing, chain signals allow the non-intersecting rails to remain separated while ensuring that trains do not stop in the middle of the junction.

They are also effective on bidirectional single-track lines with occasional passing sections. Without chain signals, a train may enter a shared stretch only to find that it cannot leave because another train is already occupying the exit path. Placing chain signals before the shared section prevents this by refusing entry unless the train can clear the segment. If several chain signals appear before a regular signal, a train waits at the first chain signal when the block beyond the regular signal is occupied.