The one-way signals are for directing traffic in a specific direction. By directing trains as shown below, you can have a much more efficient rail service.
Here I have four trains running on this one line making much more money than just one.
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Notice that the trains are traveling in both directions like a highway.
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Two-way signals can direct traffic in two directions, but they're usually best for complex stations like this:
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Normally I would give you a link to the signals guide on AMI Trains, but it is currently unavailable.