There's nothing open about either system really. However, the patents will soon expire on both systems and we'll all be able to roll our own and market whatever we want.
One thing no one can ever do, except Lionel and MTH for TMCC and DCS respectively, is market a product called TMCC or DCS. It would have to just say "RPLST8-Controller - compatible with MTH and Lionel command systems." "MTH is a trademark of MTH Inc. and Lionel is a trademark of Lionel LLC" or whatever.