Barking

The joke only works if you know the District line: three stops before Dagenham is Barking.

Counting west from Dagenham Heathway on the London Underground's District line gives Becontree, Upney, then Barking, which supplies the pun on barking mad. The formula is flexible: Green's Dictionary of Slang records the family from a 1999 Guardian use of two stops on from Barking, and two, three and six stops all circulate. Both stations sit in the same east London borough, Barking and Dagenham.