Do you mean to say that the newest GT version is actually dependent upon having Thaumcraft installed?
(Also, the fix to the problem should be to just delete the Thaumcraft folder from the Railcraft .jar file.)
Just to let everyone know I tried to delete the Thaumcraft folder in the Railcraft jar and it made Minecraft crashed with " java.lang.NoClassDefFoundError: mods/railcraft/common/carts/EntityLocomotiveSteamMagic" without Thaumcraft. So right now either we stay with GT 5.02d with Railcraft. Remove Railcraft, or install Thaumcraft. I can only request Greg (please) to possibly to add a config to disable the Thaumcraft research in GT as a temporary fix. I know the issue is with Railcraft but who knows when it will get fixed.