Cab of 999600, showing the video camera and laser measurement equipment.
It was built in 1987 using the same bodyshell as the Class 150/1 "Sprinter" units that were built from 1985-1986. It was originally classified as a Class 180 test unit, but since this designation is now used by the new First Great Western"Adelante" units, it has been reclassified into the departmental series. The unit is formed of two driving motor vehicles. Individual carriages numbered as follows:
The unit is currently painted in a plain overall yellow livery with Network Rail branding. It mostly works on lines also used by Sprinters and Pacers where the track quality is not good enough for larger and heavier track assessment stock. It is able to operate over most railway lines around Great Britain. The unit is based at the RTC in Derby.