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Oneway or Twoway Presignals?

Signals have two sides, in the basics part, the other side wasn't important, since trains can't turn back, etc. etc... But in more complex things, a badly placed twoway combo presignal can make errors in your network.

Part 1: The Station entrance problem with two entries

A RoRo station with two Entries

In this station the combo presignal can make errors, but can you notice why? If presignals F-H are green then upper-right side of combo presignal C is green too. Even if presignals D and E are red, presignal A remains green. The problem is simple: Presignals calculate signal blocks, not the choosable tracks: trains cannot reach platforms F-H from A through C, but presignals CANNOT see this!

The Error: Signal A remains green

How to fix this? Just change the combo presignal C to oneway (shown in first picture) or add an oneway combo presignal just after it, like in the next picture:

Now an oneway combo presignal is added after it

With this layout, entry presignal A can't look for platforms F-H.

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