Westbury is also pretty good for 59s, and you've got more chance of seeing them on a standing start there too, which while they're kinda 66s they've got the oompth so sound more impressive on heavy eastbound loaded stone trains.
Westbury also has Colas 70s, although I have yet to go there to see this for myself. Not been there for a few years now, I wonder if the cafe is still going there.
Acton Yard will also have 59s and 66s, which you can view from Acton Main Line station but you need to be there at the right time if that makes sense. Trains passing between Ealing Broadway and London Paddington will also give you views of what's there, make sure you're on the left hand side when heading towards Paddington. This does run the risk of a train passing at just the wrong time mind, so be prepared for a repeat run!
Stratford is good for freights too, the occasional 37 also produces on stock moves. 86s, 90s and 66s, I think also 70s, work freight here.
Carlisle overnight is legendary for freight!