The upside of rail is that it's about two hours from London, and although you may have some queueing for departure at Paddington, once you're on you are guaranteed a seat.
When you get to the other end, there's an element of luck with the shuttle buses. You can jump on one, and be straight to the festival in 20 minutes, or wait an hour or two because they are all stuck in queues (as happened this year on the Wednesday).
Here's what I'd do:
- Aim to arrive in London on Tuesday morning, or earlier. Pick up things like a tent there. Much better value than on site.
- That afternoon, Train to Bristol, bus to Glastonbury Town. Or train to Castle Cary, taxi to Glastonbury Town.
- Camp in a pre-booked campsite there, e.g. Isle of Avalon.
- Bus to festival site first thing Wednesday. Research this in advance - you'll need to know where it will go from, times etc. Locals on the ground probably won't know, transport arrangements change every year.
Doing that means you should be able to avoid the queues which will build up during the first day. Normally they aren't so bad, but this year 100k turned up on Wednesday, rather than the usual 60k. Everything got overloaded - roads, gates, everything.
The time to really start thinking about all this is around mid March. It's then that the rail fares and times start to get published. It's also possible that details of things like the Glastonbury Town shuttle might be known.