(02 Sep 2015, 4:10 pm)Dan wrote I've never actually been on the Metro to St. James before - I've always bailed at Monument. It's the only section of the network I haven't actually used (had no reason to).
I think the popping sound is caused by some sort of braking mechanism. You occasionally hear it near level crossings and such but, obviously, the tunnel makes it a bit louder. I've always been mesmerised it for some reason. You should make a journey up there sometime.