asked Her Majesty's Government:

Whether a category C or C+E light goods vehicle licence is required to drive a heavy locomotive which was first used after 1970 when it is (a) used solo without a trailer; (b) used with a trailer which is not a goods vehicle; (c) used with a trailer which is a goods vehicle, but is unladen; and (d) used with a trailer which is a goods vehicle and is laden.[HL6593]

A heavy locomotive manufactured on or after 1 January 1960 is classified as a large goods vehicle and if it is driven either as a solo unit or drawing a small trailer it requires a category C licence. If the maximum authorised mass (MAM) of the trailer exceeds 750 kilogrammes, a category C+E licence is required.

Whether the trailer itself constitutes a goods vehicle, and whether the trailer is laden or unladen, are not relevant considerations. The definition of MAM includes the weight of the trailer plus the maximum load it can carry.