Always test in conventional to see if the engine at least runs. If it doesn't, nothing to do with DCS. So then it either goes back, or you look inside.
You could try rocking the switches for DCS and 3 rail back and forth a few times. They don't always work right.
Thanks. Ok so I fired it up on the conventional Christmas layout and it does run in conventional. How do I locate the switches for conventional/ DCS? I tried again on my full layout. Other locos are all running fine in DCS so I don’t think it’s a system error.