Let's consider this. Using the light bulb analogy, if you had a 1 hp generator producing 700 watts and six 100 watt light bulbs, no problem. But if you tried connecting the same six bulb load to a 250 watt generator, it would fail due to an excessive load. You could replace the six bulbs with 40 watt version to lower the load to within the capacity of the generator. Unfortunately, you can't trade traction motors as easy as light bulbs. You could reconnect the same six light bulbs (or motors) in series parallel to reduce the load to within the capacity of the generator.

If overloaded, the load regular should go towards minimum field, but the minimum excitation level might still be too high. Modern excitation systems have a wider range of control and enforce a maximum current limit for excitation.

If you ever measure or have seen the specifications for resistance through motors, armatures or their fields; there is very little resistance. An un-energized motor only has the normal resistance of the wire itself. If you were to hook a power source to it, it should be nothing more than a direct short. But once energized, the field and armature windings create magnetic fields which create a resistance to the flow of the electrons through the wire in the windings.

According to Lenz' law "The induced EMF in any circuit is always in a direction to oppose the effect that produced it." Faraday's law says "The EMF produced in any circuit is dependent upon the rate of change of the flux linking the circuit". "... Counter Electromotive Force is directly proportional to the speed of the armature and field strength. That is, the counter emf is increased or deceased if the speed is increased or decreased respectively; the same is true if the field strength is increased or decreased." Source: Basic Electricity, Bureau of Naval Personnel. Heat does also cause increased resistance, but cemf is the primary source of the increasing resistance resulting in increased voltage and decreasing amps on traction motors as speed increases.

As far as Mr. Lemp, he actually had two separate patents that are important to railroad gas/diesel-electric propulsion - First in 1914 was the load regulator that adjusts the electrical load to prevent overloading and underloading the prime mover. The second patent in 1924 was the exciter to balance the output of the main generator. Most GE and Westinghouse equipped locomotives use either the exciter or both systems. With or without exciters, most generators still apply some of the principles of the 1924 patent in their excitation fields. (Diesel Railway Traction; "The Early History of Electric Transmission Control" 1952)