Steve,
All you need to "run" TMCC/Legacy/DCS/Conventional engines is:
TMCC or Legacy Command Base
DCS TIU
DCS Handheld Remote
Transformer with 18 volt output (I'm using a Lionel PowerHouse 180)
and various cables to run between them
This setup will run engines from Lionel, MTH, Weaver, Atlas, 3rd Rail, and Williams By Bachmann.
I have a 12x12 layout and can do a lot switching and can also run loops by putting in the diagonal liftout section at bottom left:
I only have 1 conventional engine left now, a MTH RS3 with Locosounds (which I plan on upgrading to either PS2 or ERR Cruise with Railsounds 4). Once I got away from having to tap the direction button a few times to change direction I realized how much more fun command control was, plus you'll have the ability to slow your engines down to a crawl. Even the RS3 (running in conventional) can be slowed down to under 5mph. Having to cycle thru F/N/R on conventional engines, to me, is a PITA, I've never been confident that the engine cycled like it was suppose to and have had more than one occasion that it didn't.