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Not overly busy, especially if you get there early. When I was last there on a Friday two years ago, it was noticeably busier from mid afternoon onwards, I'm guessing a few half days off holiday were taken by bashers. The late night trains were packed.
If you want to window hang, then you should easily find a window in the first couple of carriages.
Went yesterday and even though it was busy, furthest back I ended up was front of the 3rd carriage and even then I found a closer window as the train moved up the line.