Someone I know is a train conductor for the NY subway system. (For unfamiliar readers, this type of conductor does not collect tickets, but opens and closes the doors is the main person responsible for passenger safety - moreso than the train driver.)
Let's say the conductor is working his scheduled route, but the train scheduled is delayed, so that he is still on the train when Shabbat arrives. AFAIK, the MTA does not have a means where the conductor can request a substitute to take over mid-route. The assigned conductor must complete his route. If the conductor abandons the train, not only will the train not continue to operate, but passenger safety may be risked. (In NYC, people anger somewhat easily, fights occur on the subway; people get sick on the train, etc. need more ideas?)
Trains are placed out of service if there is a mechanical problem, the train itself is unsafe, or there is civil unrest on the train and it becomes more feasible to transfer passengers to another train. I don't think the MTA allows conductor religious practice as a reason to place a train out of service.
Must the conductor leave the train or does he have to complete the route to maintain passenger safety?