You are driving in stop-and-go traffic, and are approaching a railroad crossing;
a fin okay to stop on the tracks if the traffic stops. if a train approaches, the vehicles ahead of you
will move forward so you can drive off the tracks.
b: you must not stop on the tracks. make sure there is enough room for your vehicle on the other
side of the crossing before you start to cross,
c: if there are no trains in sight, you can safely stop on the tracks if the traffic stops. the traffic will
start moving again before a train can arrive

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