Re: Air tank size

Older Westinghouse books did recommend reservoir sizes for the different brake equipment generally based on brake cylinder sizes and no doubt other operating characteristics the road would provide them with.
However, don't forget that FRA has a recommended main reservoirs capacities in cubic inches.
Number & size of air compressors means faster charging & recharging of auxiliary reservoirs on the cars. That is one reason that the Time Table Special Instructions limited the cars (or tonnage) to be handled on the various grades on both the standard & narrow gauge portions of the railroad.
While waviers could be obtained from FRA at one time, on main reservoir capacity, doubt it would hold today. It might pay to review the capacities based on the freight or passenger requirements.
Also, Don't forget, if I recollect correctly, wasn't the standard Brake Pipe pressure for years on the NG 70psi? If so, it would be common for a 100psi +/- main reservoir pressure, 110 in holding (& lap?) with a double top governor. That is easier on single or double 9.5" pumps. They get beat-up pretty bad on the higher MR pressures.
One of the main reasons also for retainers on the cars. Just think if you had BP pressure maintaining capability on the engine - no leakage to contend with.
Remember in Lathrop's book when they starting getting the 8.5" cross coumpounds? How the engineers were thrilled with the air supply?
Try running a train on on just one 9.5". You'll wonder if the air will ever come up.
Just more info to ponder.
Chris