It's probably worth pointing out what a "frog" is (And when the b'ys go out for to fix the track with the gauge and the old claw-bar,/Oh I'll hear the click of the frog in the switch,/ Singin' "Jerry, go 'ile the car.") The frog, also known as the common crossing (or V-rail in Australian terminology), is the crossing point of two rails. The rhythm of the train on unwelded track is broken when the train runs over a switch ("points" in Br. usage).

I used to work in London in the 60's and lived south in Sussex, necessitating a train journey each way of about 45 mins. I could go to sleep coming home from London and wake up exactly as the train pulled into the station by my unconscious remembering the distinctive break in the rhythm caused by the frog.

Jon Bartlett