In Content Manager, right-click and choose Open configuration file for editing.
This will open the config.txt file in a text editor such as Notepad.
Look for the asset. If this is a bogey, you're stuck, but there's also a possibility this can be replaced if it's something like a horn sound or engine sound and these can be easily replaced with one that's built-in or found on the DLS.
Check the built-in or DLC you have installed. There are lots available.
Once you've chosen one, write down the KUID.
Back in your text editor, replace the missing KUID with the one you chose. You'll need to do this twice - once for the reference and the other in the kuid-table.
Once the config.txt is updated, close the text editor, and submit the asset. (Right-click and choose submit).
If all goes well, you're locomotive will run fine.