When the engineer is given the signal to move forward by the conductor he gives two short toots. This is an acknowledgment that they he has received the hand signal that it is safe to move forward.
If the conductor requires the engineer to move backward, the hand signal is acknowledged with three short toots.
When the train starts to move or slows down to stop, it is required to ring its bell until fully stopped or well underway in its movement.
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Engineers also make judgment calls on whether to sound a warning whistle if someone or something is too close to the tracks. A series of short loud bursts is a warning for people or animals to move away from the tracks.