[Pete Thornton]

I do not want to purchase a transmitter for each locomotive.

RCS is also capable of running both, and at roughly the same prices, though reversed in favor of the live steam models.

Ken,
I can recommend the RCS approach; I have two systems in my battery+r/c locos and I have friends that run the live steam version and it works as advertised with no glitching. However, it is designed for one-tx-per-loco, so it doesn't quite meet your specs.
Are there inexpensive (under $100 US) 2.4gHz recievers to control a battery locomotive? Can either 75mHz or 2.4gHz systems handle a single transmitter working with any one of a number of recievers (and if so, how many)?

The Spektrum DX6i mentioned is pretty close to your requirements. The transmitter can bind to any one of 10 receivers. The rcvrs are around $40-50 and are very small. I just installed one in a Ruby and I'm very happy with it.
To control a battery-powered loco with a conventional (aircraft-type) r/c rig like the Spektrum DX6i, you have to go to the r/c car section of the showroom. They have high-power motor controllers that will plug in to a conventional rcvr. The problem I found (a few years ago) was that they are mono-directional: like planes, race cars only go forward! However, with the 6-channel capability of the DX6i, I think you could rig up a reverser to the "gear up/down" button. Or, if you look around, there may be a motor controller designed for a tank that can reverse.

(Edit) Yes, I thought so. The off-road crowd have reversible motor controllers.
http://www3.towerhobbies.com/cgi-bin/wti0093p?&C=MJD

 

[Pete Thornton]

Tony,

Forgive me for not being conversant with your latest-and-greatest products.
About the only drawback using the Spektrum R/C is operating multiple locos at the same time independently on the same track. I am not quite sure how you would do it.

I'm not sure why you would want to do it. Gang them up as a multiple unit - sure. But running "three locos simultaneously on the same track" sounds like a nightmare. Maybe, if you had a huge loop of track and wanted to run 3 trains you could sit with your favorite beverage and occasionally adjust a train so it didn't catch up with the one in front? [Personally, I'd be sitting with my beverage dreaming up a complete control system to keep the trains separated automatically.]

I've had the pleasure of trying to manage a double-header with two widely disparate locos, and it is a very difficult juggling act. I don't plan to repeat it very often.