I originated this thread, so thanks, everybody, for the responses. I have several engines, all DCC-equipped, so it seems I need to install a decoder in my unit B. I've never installed one, but it can't be too difficult.

My HO layout is strictly a floor layout; I get the equipment out when my grandson comes to visit. He is only 8, but he enjoys the challenge of running several trains. Me too.

I recently purchased a Bachmann Santa Fe Unit B (no. 11802). There is no indication that it has a DCC control unit within.  The engine that it presumably would run with, however, does (60102)

I presume that I would have a conflict if I tried to program the Unit A.

Does this mean that Unit A and unit B run at track speed  only?