The Magical Subway Do-Si-Do

October 26, 2012 1:02 PM

Apologies in advance for a very niche question . . . I commute to and from work on the BDFM trains in New York City. I've noticed that between the Rockefeller Center stop (47-50th St) and the Bryant Park stop (42nd St) going southbound, the express trains (BD) switch sides with the local trains (FM). In other words, looking in the direction the trains travel heading south, at 50th st, the BD trains are on your right and the FM trains are on your left, whereas at 42nd st they are reversed. I've also seen both trains leave the station at the same time heading south and somehow switch sides between the two stations. My question is, how? I could see a switch in between the stations, but again, I've seen both trains leave at the same time and arrive at the same time, so that would seem to rule a switch out. Does one go above the other one? It's such a short distance between the stations I find that hard to believe, but that's the only explanation I've come up with. Am I missing something? Is there a switch and I just missed it? Is there a place online to see maps of the actual subway tracks underground?
posted by slide to travel & transportation around New York, NY (10 answers total)

There's a little overpass!

See here.
posted by cinoyter at 1:04 PM on October 26, 2012 [1 favorite]

Yep, one goes above the other. This is the case for many tracks in the system. On preview: what cinoyter said.
posted by greta simone at 1:08 PM on October 26, 2012

That's awesome - I don't even feel any elevation change on either of the trains. Thanks for the quick responses!
posted by slide at 1:11 PM on October 26, 2012

(more detailed explanation: here)
posted by millipede at 1:13 PM on October 26, 2012

The tracks north of the [Rockefeller Center] station run on three levels to form the flying junction at 53rd Street connecting the IND Queens Boulevard line into the 6th and 8th Avenue trunks. To accommodate the track arrangement in the junction, the southbound platform has the positions of the "express" and "local" tracks reversed. Trains entering 47th/50th Street station from the west (from Central Park West) enter the station on the westernmost of the downtown tracks; trains entering from the east (from Queens Boulevard) use the eastern downtown track. Between 47/50th Streets and 42nd Street the southbound tracks cross over each other (not at grade). [source]

The arrangement described north of the station can be seen in this map, along with the reversal south of the station.

On preview: I'M POSTING MY ANSWER ANYWAY.
posted by stebulus at 1:15 PM on October 26, 2012 [4 favorites]

The slope is too gentle to perceive, but yes, the F/M tracks go underneath the B/D ones. It is known as "straight rail" despite the obvious curving necessary, because there is no switch involved. The reason for the odd construction is that since the F/M come in from Queens, it is actually shortest and most direct to lead into the "left-hand" (as it were) track at Rockefeller Center. A switch actually would have been necessary to make the F/M arrive on the right-hand side. And requiring the movement of a switch on lines with such short headways would cause intolerable delays.

NYC subway track blueprints exist, but are available only to authorized persons.
posted by RRgal at 1:20 PM on October 26, 2012 [1 favorite]

I should add that it is indeed possible to make the F (but NOT the M) arrive on the express (B/D) tracks at Rockefeller Center (there is a switch south of 57th St. on the F line), but this is done only in emergencies, as it would indeed cause huge delays on the B/D lines. There is, however, no way to make the B/D arrive on the local track.

I have the authorization referenced previously.
posted by RRgal at 1:37 PM on October 26, 2012 [4 favorites]

This is all fantastic - thanks for the followup answers and for the more detailed links. I love the nuance and detail in RRgal's answers. And the enthusiasm in stebulus's.
posted by slide at 2:08 PM on October 26, 2012

This has been bothering me in the back of my mind for years! Wow, thank you for asking this, and thanks for the answers.
posted by RJ Reynolds at 7:15 AM on October 27, 2012

Thank you for asking this. I was wondering this while on the train last week, as I have many times before, but never thought to look it up.
posted by zvs at 7:54 PM on November 1, 2012