Back to the topic:
I guess everyone is now convinced that EMD E-series passenger locomotives contain two diesel-electric power plants, each of which provides electric current to both traction motors of one truck only? Good. Then, let's look at how this worked in pre-Amtrak passenger service.
Most mainline passenger trains pulled by E-units had two -- sometimes three -- units. Union Pacific used up to six units, and SP and Burlington used up to four, at the end of their respective passenger service, because they combined trains on some routes. but two or three was normal elsewhere.
A few (short) top-tier passenger trains and a sizable number of secondary trains ran with a single E-unit.
Here's how this worked in real life. The three most common en route locomotive failures are:
- A high voltage ground fault, usually -- but not always -- in a traction motor. This condition trips the ground relay, interrupting electric traction current and causing the diesel engine to drop to IDLE.
- Low water, which trips the low water alarm and shuts down the diesel engine.
- Low oil, which trips the low oil alarm and shuts down the diesel engine.
With a single EMD E-unit, any of these three en route failures would only shut down half of the power, as the other power plant would continue to produce electric current and feed it to the other truck. The passenger train, which we must remember ran on a schedule, continued moving, and the locomotive could be repaired after the train arrived at its destination, or, could be replaced en route at the next crew change point. With multiple E-units, the reduction of power would be proportionately smaller, and even less of an issue.
In comparison, if the train was powered by one Alco PA*, or one FP7, or one GP9, and one of the three most common en route failures occurred, then the passenger train and its schedule would be frozen in time, stopped, and blocking a main track. A rescue would be required. The schedule, which is the reason the railroad bought the locomotive and cars in the first place, would be ruined, and the trip would have failed to provide the advertised service to the passengers who paid for it to run.
* With the Alco-GE PA series, the two mechanical shutdowns would stop the train's progress, but the ground fault might not do so. The high voltage ground would be worked around by resetting the ground relay and using the traction motor cutout switch -- something the EMD unit did not have (or need) -- to cut out either the No.1 or No.2 truck, whichever had the grounded traction motor, by trial and error. If each truck had been independently cut out and the ground condition was not found by doing so, then the ground would be elsewhere in the system and any of the three en route failures would leave the PA and its train stopped, awaiting rescue.
