As there's not much in the way of sources of ignition or combustible stuff halfway down a steel transporter trailer and the Cab's intact I'd say it was pretty much certain one of the cars was the source of the fire.
Only other possible cause would be a brake binding and a tyre on the truck catching fire .... truckers aren't often dozy enough to let something like that get out of hand and the fire extinguishers they carry are meant to deal with that .... EV battery fire not so much!!
K
Last edited by Image; 08/12/22 03:30 PM.