I strongly suspect that if I had a tether cable cut short, with the wires to and from the rectifier linked together and any other wires isolated, the loco would run quite happily without a tender, on 12v DC, with no other changes.
I could then pair it with the “dumb” tender I’ve already produced for the Berkshire (because that came to me with the roller connections already incomplete, for the same reason, I’ve just tidied them up) and I’d be off and running for running days.
My own test loop and just-begun small O27 layout use entirely conventional Lionel KW transformer running from a 240v/110v stepdown transformer.