"A few months ago I was at the Bedford/Nostrand Ave Station on the G. I didn't see anything at the station that looked like a transfer to another line. "
Take a Queens bound G train and look out the window on your left after the train leaves Bedford-Nostrand. If you look carefully, you should see the center track descend. Where it goes? Who knows...
On the subject of tunnels and stuff, take a Brooklyn Bound Q, diamond Q or W. After the train descends from the Bridge into the tunnel in Brooklyn, look out the window to the right. If you look carefully, like maybe 5-7 seconds after the train has been in the tunnel, there is what looks like a bellmouth. It looks like a tunnel that led from some other line that was to connect to the Manhattan Bridge tracks leading to DeKalb. Anyone know anything about this mysterious tunnel, because I've heard nothing about it.