If im right in thinking, all you need to do is clone it, open the config (CCP is easyer) and change where is says "RS Steam" for example, you change it to "BR...". then commit it and there you go.
I've changed locos to scenery but the problem is you lose the bogies. It is easier to lay a short section of invisible track where you want the loco then place it. If you want to make it inanimate you could edit the config.txt file to change the engine tag to 0.