wrote

You could say the same about a journey on the tube but I don't think
anyone
would suggest installing toilets on tube trains.

The through trains between Ealing Broadway and Southend, which used the
District Line as far as Campbell Road Junction (and were available for local
passengers, e.g. between Ealing Broadway and Whitechapel) had (retention)
toilets. They ran from 1910 to 1939.

Did the Metropolitan Pullmans, Mayflower and Galatea, have toilets?

Peter