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

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Quick Facts HR X class, Type and origin ...

HR X class
Type and origin Power type Steam Designer Peter Drummond Builder North British Locomotive Company Serial number 18805–18808, 19013–19016 Build date 1909–1911 Total produced 8
Specifications Configuration: ​  • Whyte 0-6-4T  • UIC C2′ n2t Gauge 4 ft 8+1⁄2 in (1,435 mm) standard gauge Driver dia. 5 ft 0 in (1.524 m) Trailing dia. 3 ft 3 in (0.991 m) Loco weight 69.0 long tons (70.1 t; 77.3 short tons) Fuel type Coal Fuel capacity 4.5 long tons (4.6 t; 5.0 short tons) Water cap. 1,970 imp gal (9,000 L; 2,370 US gal) Firebox: ​  • Grate area 22.5 sq ft (2.09 m2) Boiler 4 ft 6+1⁄4 in (1.38 m) Boiler pressure 180 lbf/in2 (1.24 MPa) Heating surface: ​  • Firebox 120 sq ft (11 m2)  • Tubes 1,148 sq ft (106.7 m2)  • Total surface 1,268 sq ft (117.8 m2) Cylinders Two, inside Cylinder size 18+1⁄2 in × 26 in (470 mm × 660 mm)
Performance figures Tractive effort 22,082 lbf (98.23 kN)
Career Operators HR → LMS Class HR: X Power class LMS: 4P Withdrawn 1932–1936 Disposition All scrapped

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The first four were built by the North British Locomotive Company and delivered in 1909. A second batch of four was delivered in 1911.

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.

They were not popular with their crews, many thought them heavy and clumsy, and several crews had problems with water capacity. 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. The last was withdrawn in 1936.