Tjoeker is right, vehicles in Simutrans always finds the shortest* possible path, and sticks to it unless it discovers that the way has been physically removed or "permanently" blocked. The philosophy behind this is predictability. Trains won't suddenly go all the way around the world because all other paths were temporarily blocked, which could happen in for instance Transport Tycoon.
You can use waypoints to tell some trains to use one track, and other trains to use the other track. Another solution is to make one track, usually the high speed track, electrified and a little bit longer than the unelectrified low speed track.
Two-sided signals are a deadlock waiting to happen in all but very limited circumstances. While real life train controllers plan ahead to avoid such things, Simutrans has no foresight.
*Length alone is not the deciding factor. Steepness, curvature and traffic is also taken into account.