These sorts of loading errors are caused by some other mod 99% of the time (if there is only a handful of people experiencing it and the tracks work fine for everyone else). See if a clean beta install works versus your regular install. Just put the one or two extra tracks into beta and leave it stock otherwise. I had a similar problem where a car wouldn't load.
@James Nance
Most likely the problem is in the Shared folder, some mods (tracks and/or cars) come with additional files that are installed in the Shared folder, and overriding the original Coreshaders.MAS file.
Your Shared folder must be empty, except for the Coreshaders.MAS file, see piccie