Hi,
It can be a pain to locate the cause of this error message.
To make it easier to locate put a red track mark in about the center of the section of track causing the problem then tell the AI driver 'Drive to Trackmark X'.
If he happily sets off the problem is in the last half of the track section, if he still refuses to drive it is in the first half of the track.
Delete the track mark and put another track mark in the middle of the suspect section and keep repeating until you only have a short section of track to check.
Oh! Another thing to try is to to send a AI train from the other end of the faulty track section, if he can drive it OK then look for directional arrows and other signaling problems.
Lindsay