The obvious solution to the overcrowding problem on the F line is to run more F trains. The question is, Is that even possible?
During rush hours, F trains run every 6-8 minutes. Doubling their frequency would give a 3- to 4-minute headway. But on their present route, that is not possible.
They share the Queens Boulevard express tracks with the E. Which means a 3- or 4-minute headway on those tracks, and 6- to 8-minute headways for each line. Is it possible to squeeze anything more in there? Maybe. Probably not.
F trains have the 63rd St Connector all to themselves. Likewise the Rutgers St tunnel. Doubling frequencies on those segments would be possible.
The pinch point is the 6th Av local tracks, which they share with the M. The M runs every 8-10 minutes during rush hour, so theoretically, in a ten-minute span, there might be room for one more train, giving a 3-minute headway there. But where would such a train come from?
If it can't come from east of Roosevelt Avenue (actually, east of Queensbridge), there is no other possible terminal west of there. 57th St-6th Av has not been a terminal since the 63rd St Connector opened and, being only a two-track station, would not work as one now. Broadway-Lafayette is out. The next possibility would be 2d Av, if the track layout would permit, but how useful is that? After that is Jay St, and your train is already crowded long before that.
At the moment, I don't think there is a solution.
In the future, when SAS opens, it would be possible to run two lines on Second Avenue, not just the Q down Broadway as is proposed but another line—call it the Y—that would come down 2d Av, across 63d St and down 6th Av to supplement the F. (There are crossovers west of Lex Av-63d St that would permit that maneuver, aren't there?) That would double frequencies on 2d Av and add service on the line to Brooklyn where it's needed most. If you ran it express in Brooklyn, you wouldn't need frequencies as high as the F (which would remain local at all times), say every 10 minutes, and would avoid any confusion with F-express vs. F-local.
I think it would work. Just not now. Unless...
Unless the tail tracks to the Second Avenue Subway are already in place and usable. Then you could run the Y to/from Lex Av-63d St until SAS opens. (When the lower part of SAS opens and you have the problem of scheduling Q, Y, and T on the upper SAS... that's far enough in the future that I'm not even thinking about that.)
Them's my 2 cents.
Formerly of Pittsfield and Waterville (Maine), New York City, Montréal, and San Francisco.