See the picture, one way signals aren't correctly placed on curves and slopes.
This item was already reported more than a year ago and it seems unsolvable. See:
http://forum.simutrans.com/index.php?topic=146.msg12725#msg12725
normal signals are well placed, why not one way signals ???
The thread wipi linked has an answer ;)
The singals are wrongly aligned in the file. They MUST be at the entrace of a tile. Not doing so results in this behaviour.
Last time I checked, signals were aligned to exit of tile. Maybe this is because it is just a sign... ?
Well, even sign in pak 64 works more or less as intended.