Hi, I have been countering the same problem.
The train stands still and the message it gives is that there is no track / no line (I play in German so don’t know exactly what the English message is)
But anyway, when this occurs I always select the electrify-tool and follow the line the loc is supposed to go.
You have to look carefully for the little piece that stays green instead of already red. In every single time I got this message there was a small piece of electrified track along the lind the Loc had to ride.