I Think the Watercress line have done RATs (Real Ale Trains) in previous years, may be worth checking with them. The Sittingbourne & Kemsley Light railway have also done something similar too, again check with them
Shepherd Neame, a Kentish Brewer have run in the past few years, main line steam trains fron London to Faversham and also a local trip in Kent in Septemebr to co-incincide with their Beer "Hop" Festival and sometimes in May too in relation to a vinatge/classic car event, but I am not sure if these are running this year.
I did go with them several times and the real ale was being served almost as soon as we left London fairly early in the morning.