With DCS and by using the Variable output terminals on the TIU, you can run ANY 3-rail locomotive ever made in conventional mode. You can even run Lionel Legacy and TMCC in conventional mode.
In this situation, the TIU varies the voltage present on the track from ~5-18V just like a transformer would. The voltage can be controlled via the remote and (I think) the smartphone app. It can also send the bell and whistle commands that Lionel engines typically respond to. While running locomotives in conventional mode, you can continue running MTH locomotives in DCS command mode - though their top speed is limited by how much voltage is on the track.
Additionally, you don't need the Legacy system to run Lionel engines in command mode. All you need is an older TMCC command base and a special cable to connect the command base to the TIU. The cable is around $20 I think. This will let you run any TMCC loco or any Legacy loco in TMCC mode where you get speed steps remote front and rear coupler operation, crew talk and tower com, adjustable volume, etc. By using a TMCC command base with the TIU, you can keep the track at a full 18V so that everything can run at full speed (if desired).
