when I remove buildcraft from my mods this crash happens (otherwise its fine)

link:http://pastebin.com/G6aLVFGg

Only way that could happen is if you modified the jar to remove the Buildcraft API folder.

I've verified that my version of 9.4 has the requisite files from the BC API included.

I suggest that you redownload Railcraft. Hope that helps.

sorry but I didn't modify the mod files, I also redownloaded and removed any previous files made by RailCraft and it still doesn't let me on unless buildcraft is installed :(

Try with only Railcraft installed. If that works, then likely some mod is crashing during init and interrupting the mod loader's classloading routine. At which point you need to identify the mod causing the issue by testing them one by one.

its rotorycraft, I will go ask rieka about it

ok I will give that a shot

ok it must be another mod, My apologies.