This occurs in multi player. The issue is, when a non-host player gets into a train and presses C this will delete the timetable for the first train in the list.
To be clear, the "host player" is the one who is running the multi player game, all other players are "non-host players".
For example "Train A", "Train B" and "Train C" are operating fine transporting cargo to different stations. Train A is the first in the list.
A non-host player enters Train B and presses C to bring up the train timetable. This train timetable appears empty, this doesn't matter what train you get on, non-host players cannot see the timetable from within a train.
At this point the timetable for Train A has been erased. When the host player views the timetables all other trains have valid timetables however the timetable for the first train on the list is erased.
The first train on your list should be an empty train that does nothing. This stops other players accidentally breaking a train route.