Go into Content Manager and search for Missing dependencies. Anything that's missing something will be listed here.
If the route is listed, it has missing dependencies. Right-click on the route and chose show dependent assets.
A small box will open up that will list the Kuids along with the assets that are missing. If you have an unknown asset and just the Kuid number, then that's the culprit. There could be more than one, FYI.
Use the Trainz Kuid Index
www.trainzkuidindex.com to assist with the search for the missing dependency/(ies)