One of the putative explanations is that this comes from The Importance of Being Earnest. I can confirm that there is a reference to the Brighton line in Oscar Wilde's play, but the number 59 is not mentioned.
The two most famous lines from The Importance of Being Earnest are Lady Bracknell's comment that "To lose one parent ... may be regarded as a misfortune; to lose both looks like carelessness", and (hot on its heels) her response to being told that Jack (a.k.a. Ernest) was found in a hand–bag, in a cloak–room at Victoria Station. ("A hand–bag?") The cloak–room was on the Brighton line.
© Haydn Thompson 2025