If my catalog perusing was accurate, 2006 was the last year Lionel released a Legacy subway set. It was the MTA R-27 set in green and the catalog drawings looked very nice. It featured automatically opening doors. I've never seen this set in person, however. My only concern is that it was rated for O-72 diameter curves - quite large for a subway line IMO. O-54 would be reasonable. The opening doors would be nice but not necessary. Legacy control and Railsounds are the primary features I'd like to see.
I understand MTH has the subway market, but for various reasons I'd prefer a Legacy subway set.
FWIW: I don't have a subway line per se, mine is more of a pseudo subway line that travels under my elevated city and emerges at base level in a couple of places. A subway train that could be easily programmed to start and stop at the two stations would be pretty cool.