It was my second playthrough and after I beat Richter, I started jumping around and caught a glimpse of a door. Now on my third playthrough, I decided to see if i could get up there. After 30+ mins of attempts, i was able to wall jump of the flat wall and catch a tiny ledge just below the spikes.  From there, i wall jumped of the ledge and combined a double jump with a orca kick to get on the ledge.

I havent actually gone in but im posting so that other members of the community can attempt to get up there

Bruh. The door is fake.

Well i guess this can be a test of skill for people who want to do something after getting surf

Huh. By going of the edge, you can get the infinite double jump glitch and get under Flynn's train.