Rail chain signal

Rail chain signal is a rail signal variant used to control automated trains by looking beyond the immediate block and ensuring a train can both enter and exit a reserved section without becoming stuck. Chain signals are placed like regular rail signals at the right side of a track; when trains must run in both directions on the same track segment, place signals on both sides, opposite each other.

A chain signal behaves like a normal signal in that it prevents a train from entering the next block if that block is occupied. In addition, a chain signal checks the exit signals of the next block along the train’s planned path. If the relevant exit signal of that next block does not permit passage, the chain signal will stop the train from entering. When multiple exits exist from the next block, the exit that lies along the train’s current path is the one considered. An important special case is when the train’s destination can be reached from the chain signal without crossing any other signals; in that case the chain signal ignores exit signals and will allow the train through (this applies to both scheduled train stops and temporary stops).

Chain signals are particularly effective at preventing trains from stopping inside shared or crossing sections. They ensure a train only enters a complex junction, crossing, or single-track bypass if it can leave it immediately, avoiding situations where a train blocks crossing tracks or causes deadlocks in multi-train intersections.

Practical details and behavior to keep in mind:

• A chain signal with only one exit will prevent a train from entering its block if the train would have to stop inside that block (for example, because the exit is occupied or blocked).
• Trains evaluate chain signals according to their own path. Placing a chain signal before a crossing ensures a train will not enter the crossing if its chosen exit is blocked; the signal will not be influenced by other tracks’ exits that are irrelevant to the train’s route.
• Multiple chain signals before a regular signal cause the train to wait before the first chain signal if the block after the regular signal is occupied, effectively reserving the whole section only when the full path is clear.
• Signal colors indicate status: green means all exits considered are free; yellow means the block is reserved for a train and other entrance signals to that block turn red; red means all exits are occupied; blue indicates some but not all exits are free (trains may or may not stop depending on their path). A blinking chain signal indicates it is not placed on a valid rail or cannot divide the track into separate blocks.
• Common uses include preventing trains from blocking crossing routes, avoiding deadlocks at double crossings, and protecting bidirectional single-track segments with bypasses by only allowing entry when the downstream exit is available.

Use chain signals wherever trains might otherwise stop inside intersections, crossings, or shared single-track sections to keep traffic flowing and to reduce manual deadlock resolution.