The DCS/TMCC cable is not a Y cable, it's a single cable with a connector at each end. This is a clip from Barry's book, note that the 50-1018 is not compatible with the Legacy or BASE1L, and I've personally experienced this. The next one wasn't compatible with the TMCC base, finally they made one that worked with all of them. I don't know internally what changed, I can only say that the 50-1018 will NOT work with the BASE1L or the Legacy command base.
- At the present time, there are three MTH cables that may be used to connect to a Lionel command base. The three cables are:
#50-1018, TIU/TMCC 6' Connector Cable may be used only to connect the TIU to a Lionel TMCC Command Base.
#50-1007, TIU/TMCC 6' Connector Cable may be used only to connect the TIU to a Lionel Legacy Command Base or a Lionel Cab-1L Command Base.
#50-1032 TIU/TMCC-Legacy 6' Connector Cable may be used to connect the TIU to a Lionel TMCC Command Base, a Lionel Cab-1L Command Base or a Lionel Legacy Command Base.
Sometime I do actually know what I'm talking about. 
