Here are the standard whistle signals with (o) indicating a short whistle blast and (-) for a long whistle blast:

(o) apply brakes, stop

(- -) release brakes, proceed

(- o o o) flagman protect the rear of the train

(- - - -) flagman return from the west or south

(- - - - -) flagman return from the east or north

(o o) answer to other signals

(o o o) when standing, back up

(o -) inspect train for leaking air or sticking brakes

Some railroads also use optional whistle signals such as (o o o o) to indicate reduce speed and (o o o o o) to increase speed

To start a westbound train, the lead engineer calls in the flagman (- - - - -), releasing the brakes and sounds two long blasts (- -).  The helper engineer responds with two short blasts 9o o), sees the brakes release on his air gauge, and begins gently pushing the slack in as the road engine starts pulling.  If all goes well the load will be balanced with the road engine pulling a little more than half and the helper pushing the rest.  A good engineer sensed every movement of his engine and could tell from the sounds how hard both engines were working.

To stop the train, the road engineer sounded the slow down signal (o o o o) and made a light brake application to let the pusher know he intended to stop.  The helper engineer then reduced his throttle enough to keep the engine working a little until the second heavier brake application was observed on the gauge.  At that point, the helper shut down and let the engine coast until the train came to a halt.