Factorio: How to Signal a Train Junction Without Blocking the Crossing

Place signals on the train’s right side

For a basic Factorio junction, first decide the direction of travel on each rail. Signals apply on the right side of the track. An automatic train cannot pass a signal placed only on its left; bidirectional rail needs a corresponding signal on the right at that position. A physically connected track can therefore still produce No path if a section has accidentally been made one-way. [1]

Steps

1. Trace the intended travel direction and put each controlling signal on its right. If using bidirectional track, check for the required signal on both sides at the same position. [1]

Use the block overlay to inspect the junction

Hold a signal to reveal the colored rail blocks. Those colors identify the blocks, not the traffic-light state. Rail signals and chain signals create block boundaries; a train stop does not. A regular signal becomes red when the block ahead is occupied, so a red light can be normal protection rather than a defective signal. [1]

Steps

1. Hold a rail signal and inspect which tracks share one block before adding more signals. Do not rely on a station alone to separate two blocks. [1]

Put chain signals before and inside the crossing

A chain signal checks beyond its own block: a train enters only when it can proceed into the next block too. Use chain signals before and within crossings so a train does not commit to a crossing and then wait across another route. With a chain of these signals, the route must be clear as far as the block after a regular signal, or the destination station if reached first. [1]

Steps

1. Guard each approach to the crossing with a chain signal, and use chain signals for internal divisions where waiting would obstruct another route. [1]

Place regular exit signals with room for the entire train

Use regular rail signals at the crossing exits, then leave enough room before the next signal for the longest train in the network. If the train stops there with its tail still inside the preceding junction, two correctly signaled junctions can still deadlock. The tutorial gives three layout remedies for junctions too close together: use chain signals between them, move them farther apart, or shorten the trains. [1]

Steps

1. Check the longest train against the exit block. If it would foul the previous junction when stopped, change the spacing, train length or intervening signal arrangement before adding more traffic. [1]

Read a blue chain signal as a route-dependent state

A blue chain signal means the routes ahead do not all have the same availability. A train may pass only when its own route avoids the occupied block; a blue light does not mean every train can proceed. Finally, manual driving ignores signals, so the protections described here apply to automatic operation and do not make driving through a red light safe. [1] [2]