pre-signals
I shall try and explain the best I can.
At the entrance to your station to need to begin the pre-signal block. To do this, get the signal tool, put it over your existing signals, hold down "ctrl" and click. There will now be a yellow thing under your signal, this means it is a pre-signal.
Next, at the start of each platform you must make "exit signals" to do this, follow the same instructions as for "pre signals" but click twice. There will now be a white line under your signal, this is now an exit signal.
Beware, exit-signals MUST be two way signals or it won't work.
Think of it as a block, the pre-signal starts the block and the exit-siganl ends it, the trains will now behave as normal as it leaves the station.
If you still don't get it, post your savegame please
