。
Don't open the door until the train stops.
Literal
Train [subject-が] stop until, door [object-を] open must-not.
~までドアを開けてはいけない is the standard prohibition structure: [clause]+まで ('until X') + ~てはいけない ('must not do X'). まで here marks the time limit until which the action is forbidden. The whole sentence is train-safety announcement register.