So I clipped through the back of the train during the introductory of the game right before the derailment and it left me on the tracks. Cool thing is i was able to immediately find a vendor and some workstations and some easy, well equipped
enemies. The bad thing is i really want to be able to keep this playthrough but the game doesn't seem to want to let me get to Thameshaven and I may have to start over.

Is it possible to get over to Sebastian Gaunt and complete the main quest without fast travel?

If you clipped the train, and did not finish the intro sequence then your on a ticking time bomb of a glitch (game breaking). I tested it out multiple times, and had to amend my post about missing the train. Once you get close to the train wreck location at any time the game freaks out you stop dead in your tracks, the hud disappears, and all you can do is move your vision around, and that's it (a soft lock in a way). Also your save files can get corrupt too. Best thing to do is open a old save before going to the track. If the train is gone load a even earlier save, or just restart the game over completely with ALL your previous saves needing to be deleted while the game is not running (had to do that after the last test).

I bet it would have been faster restarting and running through the area you already know instead of making a topic here.

No, there is an intro movie and a scriptstart bound to the traincrash that you can´t (at last yet) skip or activate afterwards.