Technically, singular works as well. I.e a train that leaves between two others (either physically or timetable-wise) "Zug" is also a form of pull(ziehen). So

Technically, singular works as well. I.e a train that leaves between two others (either physically or timetable-wise) "Zug" is also a form of pull(ziehen). So it could also be interpreted as 'while pulling'. But that's pretty far fetched tbh I'm not the parent, however, and can't answer your question.