The Highland Railway Drummond 0-6-4T or X class were large tank engines originally intended for banking duty.

The first four were built by the North British Locomotive Company and delivered in 1909.

The design was derived from that of the class K (0-6-0) tender engines and leading dimensions were very similar although the boiler and firebox are recorded as being 'larger' but by an unspecified amount.

It seems their axleboxes were not well-proportioned either as there are reports of consistent hot box problems.

They all survived to be taken over by the London, Midland and Scottish Railway (LMS) at the 1923 Grouping.