Are you folks aware of battery remote control systems like Locolinc and Airwire? They are designed to do exactly what you've been discussing on this thread:
You don't need a DC-to-AC inverter or any exotic technology. In fact, if you build a simple circuit consisting of a rectifier, capacitor, and maybe a Zener diode to cap the voltage, I'm pretty sure you could use these systems with AC track power too. They can be installed in any can-motored loco with or without a flywheel, and make use of speed control via back-EMF (like ERR's Cruise Commander.) Airwire in particular is aimed at G-gauge so it should have ample current capacity for O gauge.
The RC is direct to the loco (there is no "command base" to buy), and I'm pretty sure the loco can be set to keep going when the signal is lost, such as might happen on long bridges or "chicken wire" tunnels. No more problems with too much or too little earth ground. No more "check track" or "RF out of range" errors! About the only disadvantage I can think of to this approach, is that because there is no central command base, it's more difficult to create a computer-controlled, fully-automated operation as you might with Lionel's Layout Control System (LCS.)
If I win the lottery and buy Roadside America, I would let my visitors run the trains using one of these systems! Also, New Jersey Hi-Railers: have you thought about this approach?