Why did some roads, in this instance the CNJ and RDG, use two single air compressors on at least some of thier steam locomotives instead of a compound compressor? This popped up in a Blue Comet forum as a question about the CNJ G-4 and I know the RDG G-1s had the same arrangment.

Off the top of my head, and with no research to back this up, I can think of two reasons:

1) Because they had them. They had single stage pumps on a lot of older equipment, a rebuild program for pumps, and spare pumps in stock at various places around their system... why introduce a new and different pump so they have to do it all again.

2) Two pumps are better than one, especially if one fails miles from nowhere. With one functioning pump, there is at least a chance that the crew can get clear of the main.

Of course, eventually the railroads realized that it was a good idea to have two cross compound pumps on an engine. This satisfied the requirement for redundancy, and they just bit the bullet, so to speak, about the issues of rebuilding and stocking a second style pump.

My thinking is that a passenger engine pulling a short commuter train did not need as much capacity as a locomotive pulling a long freight so a single lung would be enough. Having two provided a level of protection that if one died the train would not be stranded out on the mainline shutting down the entire morning commute.