If you have to pass some water
Do it in the place you orter
Please don't use my hat and be a pal
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If you found you hat a urinal?

I also learned the first verse as:

Passengers will please refrain
From flushing toilets while the train
Is standing in the station; I love you.
When the train is in the station
We must urge you constipation
If the train can't go then why should you?