But what do you mean by how fast does air travel in the train line?
Recharge or application?
DM&IR drawbarred 4 car setss together and eliminated the intermediate gladhands to cut leakage in very cold temps.
But emergency application doesn't rely on removing the air from the end of the brakepipe. George Westinghouse added a valve to his triple and revised the engineer's valve in the Summer of 1887 in order to propagate a wave of pressure at the speed of sound [about 1700 ft/sec] to activate triples in a hurry, plus exhaust the air into the auxiliary reservoirs on each car
That was later improved to be his quick service triple that performed similarly for non-emergency conditions.
There's a lot more to the AB system, but the principles set by Westinghouse, and Westinghouse & Galton must still be respected [ or worked around] by brake folks