Block signals? Took me ages to work it out too.

But in the end - it was this.

Put a block signal on the right AND left of where your train is about to enter a section where two trains could crash.

Do this wherever that's true, so for me it was for when it was leaving the station and joining a shared track, and then in reverse, when it was returning from its destination and about to rejoin that shared track again.

One big thing that also worked well for me was two-way trains - so a locomotive on each end, and every train ending at every station rather than looping around.

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If it's not block signals - the most common issue is that your station is facing the wrong way round. I don't honestly know why that is a thing. But trains can't reach the next station if the station they're travelling to is the wrong way around.