Network Rail operates a fleet of assorted locos and rolling stock which patrol Britain's railways, keeping everything oiled, greased, weeded, bolted down and pointing the right way. (At least, that's the plan)
These workings cover the entire network so even the Kyle line gets a visit from time to time.
950001, the Track Recording Unit, usually appears twice a year and loco-hauled technology can turn up too, adding a bit of variety. Network Rail's own, yellow 31s have been unusual visitors a few times and 97301 sneaked in and out under cover of darkness in January 2011. Quite commonly, 37s will be hired-in; EWS machines were used and, more recently, DRS have supplied a few.
MPVs are used (VERY occasionally) as weedkiller trains and tampers and various other bits of on-track plant wander up the line from time to time.
Other infrastructure work, such as ballast or rail replacement is usually handled by DBS 66s as far as Dingwall. Once onto the branch itself, 37s are used, which would need to be hired from DRS or WCRC nowadays, as DB Schenker no longer has anything as useful as a 37 in it's fleet.