Barry Salter wrote

At a guess, the train actually terminates at Slough, but is a
stopping
service, so it's advertised as terminating at Langley to make
journeys
faster.

A similar thing applies at Kings Cross, where the slow services to
Cambridge are advertised as terminating at Foxton.

And at Waterloo, where Weybridge via Staines services are advertised as
terminating at Addlestone (and the return journey from Weybridge as
terminating at Putney).

(though these are short/long route rather than fast/stopping service
choices).


--
Mike D