There's plenty of info about the engine if you search,
A Scania lorry engine and chassis was used as the power unit within the tender.
It's said to have cost at least a million to build and as the link below states it was used for stunts on a heritage line,,, before it all went downhill.
The steam locomotive weighed in at about 40% of a real ‘Brit’ and had a Scania lorry engine in the tender for self-propulsion. The Norwegian carriages were out of gauge to pass through stations and had to be lifted to allow filming on various sections of the line. The train was required to separate in section and the actors including Tom Cruise and stuntmen were required to be on the carriage roofs in an action scene travelling at 40 mph. They wore discreet harnesses and were attached to roof mounted running lines.
ORR's heritage railway safety team has helped make possible what appeared to be a dramatic action sequence in the upcoming Mission: Impossible film. Steve Turner, principal inspector of railways, explains more.
www.orr.gov.uk