Also, don’t rely on “it will occur to the train to do X”.
If you have a double track, it may happen that the line uses both tracks. Or not. Better force them with signals, as simonmd said.
Lines are fixed to the tracks. Trains will never ever leave their assigned track. If the track ahead is blocked by another train, a train will never switch to a parallel line by itself.
Regarding your original issue: I’d guess that it’s a problem with electrification, then. It’s not easy to spot it, but often there are tiny stretches of track that don’t have catenary, esp. near switches or stations.
