If the asset is not built-in or DLC, you can rename the asset in Content Manager.
1) At the Launcher, click on Manage Content.
2) Look for your locomotive in the listing - you can search for the name by clicking in the search window on the right.
3) Right-click on the asset you want to rename.
4) Chose Edit Config.txt
This will open in Notepad or some other text editor you use.
5) Find Username.
6) Within the double-quotes (","), type in your new name.
7) Save and close the config.txt file.
8) Submit the asset in Content Manager.
Now the best practice is to clone the asset so you have your own locomotive.
Starting with Step 2 above.
1) Highlight the asset.
2) Press CTRL+D keys at the same time to duplicate (clone) the asset.
3) Where it says Installed, scroll that list to show open for edit.
4) You will have to blank out your search information.
5) Look for the most recent open for edit asset you have in case there are others.
You can tell by the KUID number, or if you want, add in Date Modified column if it's not there by right-clicking on the columns. You may have to rearrange and resize the columns to suit.
6) The rest of the steps apply as above with the edit, save, close, and submit steps.
Hope this helps.