Okay folks, I wanted to post an update. There has been a lot of yeses, nos, and maybes for this question so I sat down (Well stood actually.) and tried this for myself. I had a 3-4 foot long board with Atlas track on it. It was started to become a diorama someday, but never got anywhere. There is one switch with a spur, but plenty of room to shuffle two smaller locomotives around on it.
The command systems were the NCE 10amp DCC command system, and a Legacy base. The DCC was wired to one outer rail and the center rail. The Legacy base to the other outer rail. The DCC was powered by my ZW-C with 180W Powerhouses (Which saw the Legacy signal in this setup BTW.). The 10amp DCC booster was adjusted from the factory 16v to 18v at the track, and was configured for common rail layouts instead of the factory double gapped layouts.
The locomotives were a Lionel 0-6-0 Dockside I converted to DCC with a ESU LokSound L V4.0, K-Line TMCC Big Boy and GG1, and Lionel Legacy S-2 electric, B6sb, and 4-6-0 Ten Wheeler. The Dockside was on the track the whole time as I swapped out the TMCC/Legacy locomotives.
The entire time the DCC locomotive ran just fine, wasn't bothered by the Legacy signal at all. I tried the other locomotives in the following order;
The S-2 seemed to work just fine, no weird feedback sound in the speaker.
The GG1 started off acting like I expected, no sounds until start up, but once I gave it a command to move, even at speed step 1, it took off at full speed. It did this forwards and backwards.
The Big Boy started sounds as soon as there was power to the track, like it would in conventional. But otherwise it ignored all commands from the Cab-2.
The ten wheeler did have some speaker buzz (The 1st one I heard it on.), but once the sounds started you couldn't notice it. It seemed to operate as expected. This was also the 1st locomotive I tried the electrocoupler on (didn't do it on any of the other locomotives up to this point.). It would not fire. It didn't buzz or anything.
Last was the B6sb. The speaker buzz was louder and could be heard while sounds were playing. It also operated as expected. The front coupler fired without any problem. The rear coupler was like the ten wheeler and would not fire. But this one as soon as I turned the power off to the track the coupler did fire. As I took it off of the track I noticed the coupler was very hot.
My final thoughts on the matter are it seems TMCC locomotives do not work with DCC, but Legacy locomotives mostly do. The speaker buzz could probably be fixed with a filter, but the electrocoupler not working and getting hot worries me. I do not know if it only started heating up after I tried firing it or not. being that the front coupler of the B6sb did work, but the rear couplers of both steamers didn't work seems odd to me. I would tend to think that there must be a difference between the front and rear, but it may be possible to mod the circuit to get the rear to work. But as it stands now, I don't think there would be any long term damage except possibly for electrocouplers to having TMCC/Legacy locomotives on the track with a DCC signal present. A Legacy signal doesn't seem to bother the DCC locomotive at all. If Lionel wanted to, I don't think it would take much of a change to the Legacy circuitry to make them run correctly on a combined Legacy/DCC layout, but being there is no real reason for them I doubt they would.
Now this is no fully conclusive test, but it is enough for me to have an answer strong enough to put this question to bed for myself. Sadly I will not be running TMCC/Legacy locomotives at the same time as DCC locomotives in the same power block. Being my layout will be wired common rail, and have two loops, further testing will have to be done to see if I can run DCC on one loop and TMCC/Legacy on the other at the same time, but that may be a year or more before that happens, unless I get bored and try it on a carpet layout in the living room. Until that time I have no plans to try any other testing. Hope this provides some answers for others.
