BART was a little early in the process - was a little stronger in the late 1960s - but there was an active Federal program to get new post-Vietnam business for aircraft and other defense companies that would be hurt by a decline in defense purchasing as the war wound down. The BART cars would never have been built by a conventional rail manufacturer at that time, and Rohr was the winner among the others. IIRC the cars did have some teething problems (as did the first round of WMATA cars in DC, also built by Rohr) structurally, but they were worked out. None meet modern transit vehicle structure requirements, of course.

Another aero company that dipped its toe into the transit game was Boeing. Remember (I know, most of us have tried hard to forget) the Muni LRVs that replaced PCCs when the subway opened? Built by the helicopter division Vertol, to Federal design requirements, and never did work right or for long. They (in both Boston and SF) have the dubious distinction of having been approved (with Federal $$) for replacement well before their 30th birthday by the Feds, which they almost never do otherwise with rail transit vehicles funded by Federal sources.

Also IIRC, both Rohr and Boeing (Vertol or otherwise) never built any significant numbers of rail transit vehicles after the BART/WMATA and SF/Boston episodes - effectively, those were one-off contracts. Rohr (per Wikipedia) is still around, though no longer independent, and apparently continued to dabble in transit stuff into the 1970s.