The book is not close at hand a the moment, but my recollection is that "The Book of Bawdy Verse" records this one as:

Passengers will please refrain
From using toilets while the train
Is standing in the station
      (We thank you)

Constipation's recommended
While the train is being tended
If the train can't go
Then why should you.

A companion piece on the same page:

Nothing looks much better
Than a young girl in a sweater
But a sweater may not be
What it appears

Boys, it's recommended
That you see what's in the sweater
Or your wedding night might end
In bitter tears.

No source is given for either of these, and the "author" is noted only as "by A Gentleman About Town," (Checkerbooks, ISBN 0-89009-411-X) as the original printing was at a time when it was not good for one's reputation (or survival) to be known for such stuff.

I vaguely recall a couple of others to the same tune in that book, but these are the only ones I believe I recall accurately enough to quote. One of the others, I think, had something to do with a statue of a famous person and ... pigeons(?), if that jogs a memory in the darker parts of someone's recollections.

John