Regarding placing signals in Train Fever, place only signals before a train pass the switch, not short after a switch because that can create deadlocks. The part of the train that has not passed the switch blockade in the example of topic starter the path of the opposing train. The switches co-operate with the signals, signals placed on parallel tracks or the part of the track from where opposing trains are situated don’t co-operate with other signals. In that case it’s the relevant co-operating switch that sets the light on green for the relevant signal(s) when the last wagon of the relevant train is passing the switch.
Regarding one way signals, in fact they don’t create strictly one way tracks, that is may-be a miss understanding. One way signals don’t allow the line path finding system to create a route in where trains pass the one way signal from its backside, thus forcing a train on the route approaching the back side of a one way signal to select another track to proceed. You can find out that one way signals don’t create strictly one way tracks if you place a junction switch behind a one way signal, allowing trains entering from the sidetrack to continue their journey driving in the ‘wrong’ direction on a so named one way track. This is not a bug but working as designed.