Well, hopefully Dallee can come up with a complete solution as tripleo admits unfamiliarity with the nuts-and-volts of train operation. So while he is pursuing that approach and this being a discussion forum to exchange ideas...
With loops at each end, if one-direction operation is acceptable, I figure he could lock engines (assume these are conventional AC 3-rail) into Forward. Then, using the string-of-diodes approach to generate multiple AC voltages, a set of timed-relays could step the voltage up and down by tapping into different points of the diode string. It wouldn't be as smooth as an electronically ramped AC voltage but could be done inexpensively using timer relay modules from eBay ($5 each). I think it would be easy to grasp how it works (no software, programming, etc.) but requires a level of DIY enthusiasm that is not for everyone...
