are you aware that if the governor has his way we will be getting just exactly what you propose, except with metro-north in the east bronx?: http://web.mta.info/mta/planning/psas/
Yeah, it's a first step using existing infrastructure.
Bringing Metro-North down the West Side is really the biggest benefit, but I don't see many riders traveling through Penn Station on a crosstown trip...
__________________ la forme d'une ville change plus vite, hélas! que le coeur d'un mortel...