Mike, originally when the first railroads to switch over from steam to diesel they generally bought 3 types of engine. They generally bought 6 axle units like the PA and E units with A-1-A trucks where the center axle was an idler for passenger power. They would buy 4 axle b-b units (all four axles powered) for freight use. Typically F-units, FA's, etc. Then they would buy Switch engines for yard work. They discovered that they had a problem with that selection of locomotive types. The F-units worked well for fast freight. The E-units, Pa's, etc worked well for passenger service, although the Santa Fe switched over to F-units early on for most passenger service because the 4 powered axles gave them better traction over mountain passes. The switch engines worked well in the yards and doing local switching, but "peddler" or local freight switching in the next town or two over was a problem. The yard switchers (sw types) were geared too low, and lacked power so they could not be geared higher to spend much time on the mainline to handle regional or extended local freight. The F-units could handle the local freight and many roads used them for a long time, but visibility to the rear was non-existent. It required 3 men to couple or uncouple a car. One man at the point where the train break was to be made, who then signaled to a man near the cab who had a visual line of site to the engineer. The engineer had to do everything "blind." The road switcher had a ong hood on one end and short hood on the other end. The first one was the Alco s-1, but since it was a longer chassis version of the s-1 switch engine without extra power, it was not that popular in the Western U.S. although it was very popular on the East Coast and I think particularly in New England. The Rs2 & Rs3 were later developments of the Rs1 with a bit more power, but not enough for roads like U.P. and Santa Fe to be satisfied. I'm not sure which was the first powerful gp type locomotive to come out. The FM H-16-44 may have been the first high horsepower road switcher. They had the power to pull a train at mainline speeds, and up grades to go farther than a local switcher taking a cut of cars to an industry a mile or two from the yard, but they also had superior visibility for making switch moves when they got to where they were to work.
The problem for the FM was that the opposed piston engine (2 pistons coming together in the cylinder with an upper and a lower crankshaft) was designed during WW2 as a submarine engine. It was later used in all sorts of diesel powered ships. In a ship they had a ready supply of salt water at low temps to use for cooling the fresh water in the engines cooling system. When they were adapted to locomotive service, they didn't have the cold sea water to cool the fresh water and tended to overheat in the Southwestern deserts. The other problem was that in the event of an engine overhaul being needed, it was a lot more complicated and time consuming to remove the upper crankshaft and work your way down to do the rebuild, than it was to remove cylinder heads in a more conventional engine. Also the more conventional engine frequently had smaller cylinder heads that would be for 2 cylinders, so in the event of a burned valve they could remove just one relatively small cylinder head, do a valve job on it and replace it. The third problem that the Santa Fe had with them was that the running boards/walkways were higher than on the Emd Gp7s and 9s. This created a problem when the Santa Fe tried to "mu" an FM with a Gp7 or Gp9. The locomotives would work together, but a crew man could not comfortably move from one locomotive to the other because the walkways didn't match. Santa Fe ran a few H-16-44s (never owned a "Trainmaster") but once they Gp7's arrived the FM's were relegated to helper service in the mountains.