Restricted Use Vehicles
Rolling stock marked with yellow headstocks are restricted use vehicles and may only be used on the main line with the authority of the Civil Engineering Manager or the Chief Engineer.
(Source: FR Rule Book)
Examples include:
- Granite wagons
- Welsh Pony (as seen here)
- K1 (It was, prior to gaining 'type approval', now awaiting overhaul after the boiler certificate expired in 2014)