If anything is open for edit, missing, or faulty, the loco will show up red and can't be used in the game.
Start Content Manager and check that the locomotive is not faulty or has faulty or missing dependencies.
If you see any missing dependencies, right-click on the locomotive and click on view dependencies. A new window will open up. Click on status and sort so that the unknown, or download appears at the top.
Download anything that can be downloaded.
If any dependencies show open for edit, right-click and choose revert to original.
If anything is faulty, list the errors here and we can assist you with the errors.
John
Trainz User Since: 12-2003
Trainz User ID: 124863
T:ANE Build: 94829
TRS2019/Trainz-PLUS: 109641