Maybe this post will save some time and hair for first-time people. The decoders only work on POM, and If you need to reset a decoder with an unknown address, there is a "broadcast" address on all MTH HO's . Set your controller to address 55 and program CV 55=55. This is in the manual, but (I think) poorly worded. 55 is only for resets; it can't be used as an engine's address. And of course, never have more than the intended MTH's on track when resetting.
But there's more, not in the manual: leave power on the engine after any programming, including reset, for 30 secs or so, or the programming may not take effect ( there must be a tiny person in there who can only work so fast ). MTH did enclose a separate piece of paper talking about 5 seconds which didn't make much sense, but I don't think that's long enough. The original address can get clobbered without establishing the new one.
In the case of my 4449, I put up with the limitations because I like MTH's steam sounds better than anyone else's.
Hal