> But what do you mean by how fast does air travel in the train line?

> Recharge or application?

Recharge and application are dependent on the rate of pressure increase or decrease, which is related to air flow rate. Flow rate is kept very low by using piping which is large in diameter (1 1/4 - 1 1/2") with as few bends as possible on each car -- long radius bends being the most recommended.

>But emergency application doesn't rely on removing the air from the end of the brakepipe.

Yes, it does: from the end of the brake pipe which is open to the atmosphere, be it a broken hose, a broken pipe, a parted train, a caboose dump valve, or the automatic brake valve being placed in emergency ("big hole" is a reference to the large diameter opening in the automatic brake valve which is opened in the emergency position).

> 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

This is quick action, and travels through a train at approximately 900 ft/sec. Since there is a tiny bit of delay in the mechanism of the valve in responding to the pressure change, it's probably safe to say that air flow rate is slightly in excess of 900 ft/sec.

Quick action is a function of the apparatus on the car responding to a sudden pressure drop in the pipe. The engineer's brake valve has nothing to do with quick action, other than to initiate it when placed in the emergency position. Revisions and improvements to automatic brake valves between 1872 and 1892 were concerned with reliability and operation in service applications and releases.

Quick action doesn't exhaust the brake pipe into the auxiliary reservoirs, rather it depletes a few psi of brake pipe air locally at the triple/control valve in order to transmit the sudden pressure drop to the next car(s) in the train, where the quick action process is repeated, and so on throughout the train. Whether or not is this pressure is sent into the brake cylinder, exhausted to atmosphere, or stored in small reservoir depends on the model of the triple/control valve.

>That was later improved to be his quick service triple that performed similarly for non-emergency conditions.

Yes, although the quick service feature is separate from quick action.

>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

Galton didn't participate in Westinghouse's development of the air brake. The best known individuals who did were WABCo employees W. V. Turner and S. W. Dudley.