In a recent post, I discussed the Regional Plan Association's TriboroRX proposal, and Michael Frumin's research on potential usage. I mentioned that it was not necessarily the best solution to run a single train along the entire route, that it might be better to chop the right-of-way up and use it to extend other lines. In particular, I highlighted the 1969 proposal to re-route the L train to Brooklyn College. I can definitely see value in that proposal: from Wilson Avenue to New Lots Avenue, the L stations are all directly over the cut of the Bay Ridge Branch; the East 105th Street station is three blocks south of the cut, and the Rockaway Parkway terminal is four blocks south. The inconvenience of moving these two stations would be more than compensated by the improved connections.
You could conceivably extend the other trains to the west of Brooklyn College; the 2 and 5 trains come to mind. The problem is that those trains already travel much longer distances than the L, taking over an hour to get all the way from the northern edge of the Bronx to Brooklyn College. The situation is similar with the B, the Q, the F, the M and the D trains, but each would require splitting one of the routes as well as extending it - or else abandoning B service to Brighton Beach or M service to Bensonhurst.
The L currently takes 40 minutes to go from Eighth Avenue to Rockaway Parkway. It would probably take another ten minutes to get to Brooklyn College, and then another 10-15 to New Utrecht Avenue. Another possibility would be to extend the G train, which takes only 35 minutes to get from Court Square to Church Avenue. It would require tunneling new tracks under the elevated F viaduct from Church Avenue to the right-of-way.
From New Utrecht Avenue west to Fourth Avenue, the right-of-way runs right next to the N train (with two unused express trackways in the N right-of-way). It does not make sense to duplicate the N service, unless the frequencies are too low. West of Fourth Avenue, there are two reasons I can think of to run a train to the Brooklyn Army Terminal: jobs and the ferry. If this is desired, then extending the G would probably be the best choice. The L would be extended to MacDonald Avenue, and the G to the Brooklyn Army Terminal.
These two extensions combined would do wonders to connect the southern parts of Brooklyn with each other, and with Manhattan. It would go a long way towards making Brooklyn College more accessible to people from all over the borough, and bring new service to the areas of East Flatbush, Flatlands and Brownsville that haven't had rapid transit in many years.