Consider this situation, all signals are Path-signals: 
Obviously, it would have been more efficient if the first departing train would have reserved the bottom track. What happened is that the first train actually broke down, blocking the other.
Also, the reverse happens. Both trains are coming from the right side of the picture, each reserving their own track, and it only messes up at the last signal, where the train on the upper track reserves the lower platform, thus completely stopping the other train.
Is there any way of making sure this doesn't happen?
Best Answer
The solution is to use one-way signals on each of those tracks. That way, you'll be forcing the travel direction on each track.
In your screenshot, I'd force left-to-right direction for the bottom track, and right-to-left to the top one. If you are British, probably you'll want it reversed.
Read about path signals on the OpenTTD wiki. There are two types of path signals: a standard, and a one-way. You'll want to use the one-way path signal.
Also read about terminal stations for suggestions on how to place signals near the station itself.