The only chance is to change the Legacy engine to TMCC mode in your Cab2. Still unlikely it will work unless gear ratios are the same. Set them up on the same track but a few inches from one another and see how closely the stay with each other. If they part ways more than 1" per foot they will fight each other when coupled together.
If you build them into a train, the TMCC engine must be the lead engine. The Legacy engines will automatically use TMCC settings as long as a TMCC engine is the lead. That being said, I would advise you to run them separated (not coupled) before assuming they'll be fine in a train. If they seem to keep close speed you should be ok. Typically I refrain from mixing them as the chance they'll work is not guaranteed.
I have the Century Club TMCC freight Sharks (6-14532) that came with both A-units powered. Any thoughts on issues with running the Legacy Powered B-Unit (6-34704) in TMCC mode between the two A-units? I understand that the TMCC unit needs to be in front, but what about a TMCC unit also behind the Legacy B-Unit? I don't want to buy the B-unit just "to see if it works" if there is little chance it will work.
Truthfully, there's no easy way to know if they would be compatible without actually trying them. I've had mixed results running TMCC and Legacy diesels together.
Dumb question, why do you need a 3rd powered unit? Four motors should be good for a lot of cars! What are you accomplishing with the extra motors?
I like lots of power Actually, I want sound in all my engines. I have been buying the sound boards for the A units and adding it to the power B units. Have not found a way to add sound to a non-powered B unit that sounds correct with the As. (Yes, you can add a wired spreaker from one of the As and I will probably wind up doing that.)
I do have an early Legacy E7 ABA with all units powered with sound pulling a 20 passenger car consist. Looks, sounds, and runs great.
Well, you can add sound to the B-Unit without adding power. Use the ERR F3/F7 sound board and an RCM2 to provide serial data, this one is only $40 and available on Lionel parts. I've added sound boards to a few of my dummies that have smoke since they already have the receiver board.
I hear you about the sounds, I have a Rock Island A-B-A E8 set that I upgraded to Legacy. It has smoke and sound in all three units, and both of the A-units are powered. I'm sure it would pull 20 passenger cars without a problem, I only had it pulling 14 as that was all that were in the yard at the time. I also did extra sounds in an F3 A-B-B-A so they all of sound and smoke, and three of them powered with the 4th being the SuperBass unit. They do get your attention.
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