One Generator Field switch per locomotive unit.
The generator field switch is within reach of the Engineer's seat, (or, on booster units, at the hostler control stand) and only the one on the controlling unit of the consist is closed ("on"). Generator Field switches on trailing switches are open ("off"). As Hot Water pointed out, this is a multiple unit trainlined feature. It enables excitation of the main generator.
EMD units did not have traction motor cutout switches until the GP18/GP20/SD18/SD24. Thus, the FP7 and GP7/9 did not have them. Even after being made available, they were optional, and Norfolk& Western, as an example, did not equip its locomotives with them, in the belief that the Mechanical forces would be forced to fix the electrical ground instead of trying to show the locomotive as available for service. Mechanical Departments cherish their availability numbers and will cheat in order to make good numbers.
EMD's first traction motor 'cutout switches' were a plastic flag right on the power contactor for each traction motor , which, if rotated 90 degrees, would block that contactor open. The locomotive unit first had to be isolated, and then 1 traction motor on 4-motor units, or one traction motor on each truck of 6-motor units had to be isolated. Then the locomotive would be placed on line and -- if the ground fault no longer tripped the relay, the correct traction motor had been found. This could result in a lot of trying and failing until the correct traction motor was found. And it always made me nervous to have my hand so close to a large contactor that had been energized with up to 1,400 volts of direct current just a short time earlier. Later, rotary switches on the Engine Control Panel, on the rear wall of the cab, were provided for cutting out traction motors. But this was all later, and only the second generation EMD passenger units such as SDP40 and FP45 and F40PH had them. E-units, F-units, and early geeps on passenger trains did not have traction motor cutout switches.
The whole point of my last post and this one -- which were not aimed at any particular person, but just intended to bring the conversation back to the very first post on this thread -- is this:
E-units were machines designed to move passenger trains and thereby generate revenue for railroads which purchased them. They were engineered at EMD with the idea that, if an en route failure should occur, the second on-board power plant would enable the train to seamlessly continue moving toward a forward location where the ailing locomotive could be replaced or repaired, with as little delay to passengers and Railway Express and U S Mail, as possible. They were not overly complicated. This technology served its purpose until the end of railroad-owned passenger train service. Around 1960, EMD equipped its freight and general purpose locomotives with devices that would allow those units to also keep going after a high voltage ground fault had occurred, and get to the next forward shop. But only E-units* could continue after a low water or low oil shutdown, and this was important to the type of service for which they were purchased.
* Yes, DD35's and other two-engine freight units could also do this, but they were a small percentage of freight units, exceptions to the norm.