The core of the best tm train Troopers team is Veers, Piet, Range. The other 2 are flexible.
Veers has the tm gains in both his lead and unique.
Piett feeds this gains by every action from a trooper granting a buff to all troopers, plus he calls at least one assist, usually 2, to increase this further.
Range makes more assists happen, feeding off both the Veers lead and the Piett buffs.
After these 3, the remaining squad doesn't really matter. Starck with zeta can reinforce the train by removing enemy TM. Gideon can get the team ahead, and is very fast to get the train started. But neither of these are really necessary with the core 3, so it can be whatever other members you want. Dark certainly gets the job done faster, and can't go in an Iden team.