I'm convinced the Hogwarts Express is busier now than it was last summer. I've regularly seen 30+ min waits in both directions.
The line in King's Cross was almost completely full this afternoon (the line stretched out into the corridor that passes the London travel posters).
If you're determined to get the complete Potter experience (riding the Hogwarts Express in both directions), you could do this:
-Start the day in USF and head straight to Diagon Alley. Do the standby line for Gringotts so that you see the entire queue.
-Go back out to London, take the Hogwarts Express to Hogsmeade. Ride Forbidden Journey standby so that you see the full queue. That might be a 45-60 wait by mid-morning.
-Look around Hogsmeade, go to Honeydukes and have a cauldron cake, or go inside Three Broomsticks/Hog's Head Pub for lunch or a drink. Either have lunch there or wait and have lunch at Leaky Cauldron when you've gone back to Diagon.
-Take the Hogwarts Express back to London, browse Diagon Alley, visit Olivanders wand shop, watch the shows (the puppet troupe and Celestina Warbeck), take in the atmosphere, and have some ice cream and/or one of the varieties of Butterbeer.
-Whatever's remaining of the day fill out with USF's attractions: Men in Black, Transformers, E.T., Mummy, Simpson's, etc. You'd miss Spider-man this way, which is a shame, but it would get you all of the Potter stuff and maybe a couple of the other headliners.
This is a big help, thanks a lot for all of your advice!