A mystery on the B/D/F line

Sometimes in Manhattan I take the downtown D train in from Columbus Circle, and when it gets to 47/50th st it's on the right hand side of the platform across from the F train which is on the left hand side. A short journey through the tunnel and we're at 42nd st, however the D is now on the left hand side of the platform while the F is on the right. The two lines have basically switched sides somewhere in the tunnel.

But I can't work out how. To cross the tracks in the tunnel would be madness, and besides if they did that then one of the trains would be forced to wait while the other one crossed ahead of it. I know this doesn't happen because oftentimes both trains will be neck and neck at 47/50th and then they pull into 42nd at the same time.

The only other thing I can think of is that one track crosses underneath the other one somewhere in the tunnel. However I've been on both trains, and I don't detect any kind of descent or ascent between stops. It feels like both trains just zip through the tunnel in a straight line. It's quite a short distance between stops so you'd imagine the incline would be relatively steep. It really baffles me. Any ideas?

bapu
I prefer BFD3.

I see you have figured out how to Jam with your Stix and avoided that Addiction in using those loony Toon tracks.  

Mesh

bapu
I prefer BFD3.

I see you have figured out how to Jam with your Stix and avoided that Addiction in using those loony Toon tracks.  

Izee wot youse dun dare.

What's a train?

Well I found the answer and apparently one track passes underneath the other one. Which is amazing to me because you really don't feel any kind of gradient at all. Looks like I'm not the only one to have wondered this. 
 
http://ask.metafilter.com...-Magical-Subway-DoSiDo

KenB123
There is a fifth dimension beyond that which is known to man. It is a dimension as vast as space and as timeless as infinity. It is the middle ground between light and shadow, between science and superstition, and it lies between the pit of man's fears and the summit of his knowledge. This is the dimension of imagination. It is an area which we call the Twilight Zone.

I thought this was the Fifth Dimension (I was a DJ from '80 thru '83 for Aquarius Entertainment and had to play this damn song once every hour - man did I get sick of hearing it!).
 

Yeah , it the underneath thing which I see you already found out  ...
 
Kenny

bapu

craigb

BobF
Do you see any Rubik's cubes laying around?

 
I once solved five mixed up cubes in an average around 39 seconds each as part of a Guiness Book of World Records event back in 1982.  Unfortunately, I wasn't the fastest though...
 
I do like to use alternative solutions however!  The pics below are one of these.  You can't solve then twist to get them either, you have to actually solve the cube in this pattern.
 


 
 

Turn the top one upside down and front to back and you get the bottom juan.
 
Takes about 1.5 seconds, no?

 
WHOA!!!!!!