J.
We've been a couple of times. It's not as crowded as it was a few years ago, but I would buy tickets in advance because it's a long drive (at least from where I am) and the Thomas train spots are limited if you want to ride on it.
It's not that insane, and I would not arrive at 7:30. We went last summer and arrived at about 11, it was fine.
I love the railway museum even on a non-thomas day. It's a lot of fun! And the volunteers (if you catch them on a less busy day) are totally fascinating and have lots of great stories about the trains. Have a great time.