Rail signal

Rail signal is a rail-side building used to divide tracks into blocks and control train movement by granting or denying trains permission to enter the next block. Proper placement of rail signals is essential for automated, collision-free rail networks: an automatic train will not enter a block unless it has a valid signal path allowing travel in that direction. Two signals placed directly opposite each other permit bidirectional travel through that point.

By default signals are placed on the right side of the track (this is configurable in options). When the player holds a rail signal, the position directly opposite an existing signal is highlighted in white to help place opposing signals for two-way operation. If a signal is only on the left side of a track segment, trains configured to travel in the default direction will not consider that block passable.

Rail signals display four visual states that indicate block occupancy and permissions:

• Green: the monitored block is empty and the train may enter.
• Yellow: a train is approaching and has approval to enter the next block; the approaching train will not be able to stop before the monitored block and therefore will pass the signal. The debug option show-train-braking-distance reveals the stopping distance used to determine yellow states.
• Red: the monitored block is occupied or another signal monitoring it is yellow; locomotives stop before a red signal.
• Blinking: the signal is not placed on a rail or the signal is misconfigured such that it monitors the same block as the block before the signal.

Rail chain signals serve a complementary role by propagating the state of the block ahead; they are useful where trains must stop earlier (for example at intersections) because chain signals will mimic the signal state of the block immediately in front of them.

Rail signals can be connected to the circuit network. A configurable condition can force a rail signal to turn red when the condition is true. In normal operation a rail signal also outputs a unique circuit signal depending on whether it is in the green, yellow, or red state. If the signal is forced red by a circuit-network condition it will not output its state signal to the network.

Practical notes:

• Place signals to separate tracks into appropriately sized blocks so trains can reserve enough distance to stop safely; blocks that are too long or too short affect throughput and safety.
• Use opposing signals to enable true two-way tracks; missing an opposite signal prevents trains from treating the track as passable in the other direction.
• Combine chain signals and normal rail signals at junctions: chain signals before an intersection prevent trains entering when the intersection ahead is blocked; regular signals after the intersection mark the start of the next block.
• Use the circuit-network forcing-to-red feature to implement manual or automated block closures (e.g., for maintenance, station control, or complex routing logic).