Thanks Mike CT and ByronB for your responses.
Mike - I've actually got a hard copy of the 'Original' TMCC Guide that you linked and I read it from cover to cover before I began trying to build a Lash-Up. That said, if you refer to the last (or 3rd) Detail listed on the top half of page 27 I've wondered what the missing wording at the end of that Detail would/should have said. I assume "Always clear a lash-up once you've broken it up" is done by 'reprogramming the Command reverse unit to restore features' (in each engine individually) that is referred to in every Lionel Owner's Manual (and to which Byron referred), but I'm not absolutely sure about that.
Byron - To your point about doing a reset, I've tried creating a lash-up multiple times, using several different engines, some consisting of TMCC equipped only and others Legacy only, some second hand and some which I bought brand new, and nothing has been successful -so far; but I may try the reset again for a few engines - just for the heck of it.
One other thing I'm going to try is the result of me re-reading (tonight) the 2nd Detail on the top half of page 27 (ie. Choose TRAIN ID#s with numbers different from PowerMaster ID#s).
My small layout consists of two independent ovals and from the time I began using TMCC I assigned #1 as the ID of one PowerMaster and #2 as the ID# of the other PM by switching to the Program position on each PM, and then pressing TR1, Set and TR2, Set on my CAB-1L - as described in the Manual. However, when I've tried to create a Lash-Up (with each engine in the 'RUN" position) the first thing I (must) do is press TR 1 (or 2) and then Boost to electrify the appropriate track, and then when I see the engine cab light go on I immediately press the TR (this time intended for TRAIN) and then select a number from 1-9; and I'm wondering whether this is causing the PM to be confused and the lack of being able to get any movement once I've completed the rest of the required TRAIN sequence and pressed the SET button. Nothing happens anyway.
Please guys, if I'm overlooking anything that's obvious to you, please let me know. All I can say is that it's a good thing I'm retired and have plenty of time to play around with things like this, particularly now that winter's coming. (lol) I'll fool around tomorrow again but am doubtful that I'll have success.