unknown asset

Is there anything that can be done about this?

I have a session that is showing as having unknown assets. I've had these before and despite them being missing the session is still playable.

However these assets appear to be the wagons attached to the train. I'm supposed to be pulling 38 cars but all I am driving is the loco. At one point I need to runaround the train which I can't so the session is unplayable.

I also don't understand why they would have been deleted (I'm assuming this is what has happened?)
Unknown assets are not found on the DLS or Built-in. They may be from a DLC package that needs to be installed or from a third-party site.