Stupid way to skin it.
For all except the completely flat battery case you have a perfectly good source of 24V in the batteries themselves (Remember you only need a few amps for the field and once the field comes up the generator will pick up the load including its own field winding).
All you need to do is detect when you have a vaguely reasonable shaft speed (say 12MPH or so) and then power the field from the battery directly (Relay or such), regulate field current to maintain setpoint, job done (Remember to protect the field switching device from back EMF when the contacts open!).
Now if the batteries are fully flat then you need some sort of boost converter to run the field up to the point that it becomes self sustaining, but that should be very much the exceptional situation, and should probably be dealt with back at the shed by charging the batteries and putting a flea in the ear of whoever left the lights on overnight. You could even have a port to plug in a couple of 12V SLA cells to power the field if you had a really flat battery and needed to run the train right now, once the thing was in motion the generator would quickly bring the main battery up to voltage).
I have actually seen video of the (modern diesel) Edinburgh sleeper needing one of its cars emergency batteries jumped from another car because the bus voltage was so low that the charging supplys would not turn on (Yep, someone had left the lights on with the carriage parked up).
Hold on, sketching it...
I think the controller is probably a TL494 or something of the sort (Or a small micro with a couple of ADCs, but that is harder to maintain long term, KISS very much applies).
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