TMCC/Legacy Command Control Hiccups

Obviously, we'd have to know more details about your layout, this is not a problem for me running multiple trains, both TMCC/Legacy and DCS, on my 24 x 12 layout.  There's obviously more in play here.

Trying to diagnose a general "it doesn't work right" is impossible, you need to have a repeatable scenario that you can demonstrate what is happening.

Lots of people run multiple loop layouts and engines with TMCC/Legacy with no issues, especially not with a different train responding when you address and activate the whistle for another train, so there's definitely something else going on.

You say all the loops are isolated ... for starters, how are you supplying (wiring) power and the command signal to each loop ?

The usual explanation for what you describe (wrong loco responds to commands) is the simple "you hit a different key than you thought you did on the remote."  If the locomotive is number 3 and you hit number 2 by mistake, that's what happens.

I have 4 loops of track. Each loop is powered by one of the 4 outputs from ZWL with 12 gauge wire.  I have essentially 4 bus bars of power, one for each loop, with multiple feeders going to each loop of track. I address the track and then address the engine ID.  Most of the time I run a mixture of Conventional and Command locomotives. Today I was running a conventional Williams ALCO and Lionel's Spirit of the Century Train (TMCC) at the same time on two separate loops (I did not run anything today on the other two loops) and it just seems like one or the other was always picking up speed. At one point the Spirit of the Century Train uncoupled. These things seem to happen when I attempt to activate the whistle control on either locomotive. This has happened previously with other conventional/command locomotive combinations I try to run. I know that sometimes I have entered the wrong ID numbers but that's not always the case. I don't seem to have this problem if I run all command locomotives or all conventional locomotives, just when I have a combination with at least one command and one conventional locomotive, each running on its own separate loop. Apparently I am doing something wrong.

You're running trains with no cruise control.  Slight variations in voltage on the track will affect their speeds.

Hmmm .....ok that makes sense GRJ, thanks !

Obviously my cognitive abilities are failing me faster than I anticipated. This is the same reason my wife controls the TV remote while streaming Bitbox.

You seem to be running TMCC and Conventional on different loops.  The loop with TMCC is running on fixed voltage (16ish volts). The conventional is running off variable track power. Maybe it could be that the power from the conventional loop is bridging to the TMCC loop.  I am not sure what that could cause when changing power (speed) to the conventional loop.  Other  members here might comment if this is a possibility.

JVM-51, yes that is what seems to be happening, although I don't know how unless it has something to do with the way I am using the ZWL. I have it set for Run, Command, 4-CHL. I use the Cab1-L/Base 1-L Set.

To operate the layout, I address a TRACK ID to operate a conventional locomotive; then I address a TRACK ID followed by an Engine ID when I am running a TMCC/Legacy locomotive. Is this incorrect ?

Actually, now I think I see what you're running into.  You're using the powermaster emulation of the ZWL and that's where things are getting crossed up.  You should be running the command stuff with full track voltage.

You shouldn't have to address the  track ID before addressing a  engine ID on a TMCC or Legacy engine. I think that would just confuse things.