Sorry this is a repost from what I put on the forums, I was unaware there was an issue tracker:

Crash Report: http://pastebin.com/25hGJ7mY

I just updated my server from 8.3.0.0 to 8.3.2.0 and everything runs, except logging in with metal posts in rendering distance will crash the client to desktop. This happens for multiple players and they can't log in until I'm able to teleport them away.

You need to update your mods, one of them is using an outdated Railcraft API.

Sorry to bug you, but do you have any idea which mod it could be?
I've been trying to figure it out to update it but the area keeps causing crashes upon login for players..
Any help would be greatly appreciated

You'll have to look through your mods. Look for a railcraft folder in the jar/zip files.
Figure out which ones interact with railcraft and check those.

(or just update everything to the latest version)