Factorio train signals versus chain signals: safe intersection basics

Understand rail blocks

Signals divide rail systems into blocks. A train must respect block occupancy; two trains cannot simply be directed into the same occupied section. Ordinary rail signals are useful for controlling entry into a block when the next section can be occupied safely.

Before designing a junction, inspect the colored block visualization in-game. If one long stretch is treated as one block, trains may wait much farther apart than expected.

Choose entry and exit behavior

Chain signals take downstream signal conditions into account. A common starting principle is a chain signal before a crossing or junction and a regular signal after it, so that trains are less likely to enter a conflict area they cannot leave.

This is a design principle rather than a magical placement rule. Complex intersections need enough space, clear directions and appropriate signal spacing; mixed one-way and two-way rail segments make matters harder.

1. 01Build a tiny single-direction test junction.
2. 02Use block visualization before adding trains.
3. 03Add a chain signal before the conflict area and a regular exit signal after it.

Debug a train waiting at a signal

Look beyond the signal immediately in front of the train. A chain signal may be waiting because its intended route through the next blocks is not free. Check train routes, station names, opposing directions and downstream occupied blocks.

If traffic is still stuck, temporarily simplify the track to a single known route and restore branches one at a time. Avoid changing every signal at once; you lose the ability to identify the actual cause.