LSWR N15 Class - Design and Construction

Design and Construction

For detailed information on numbering variations, see: Livery and numbering

Trials undertaken in 1914 with the H15 class prototype had demonstrated to Urie that the basic design showed considerable speed potential on the Western section of the LSWR from Basingstoke westwards, and could form the basis of a powerful new class of 4-6-0 express passenger locomotive with larger 6 ft 7 in (2.01 m) driving wheels. The LSWR required such a locomotive, which would need to cope with increasing train loads on this long and arduous route to the West Country. The result was the N15 class design, completed by Urie in 1917. It incorporated features from the H15 class, including eight-wheel double bogie tenders with outside plate frames over the wheels and exposed Walschaerts valve gear. High running plates along the boiler were retained for ease of oiling and maintenance.

Despite the similarities, the N15 class represented a refinement of the H15 template. The cylinders were increased in size to 22 in × 28 in (560 mm × 710 mm) in diameter, the largest used on a British steam locomotive at that time. The substantial boiler design was also different to the parallel version used on the H15, and became the first tapered types to be constructed at Eastleigh Works. Contrary to boiler construction practices elsewhere where tapering began near the firebox, it was restricted to the front end of the N15’s barrel to reduce the diameter of the smokebox, and consequently the weight carried by the front bogie. The design also featured Urie’s design of narrow-diameter "stovepipe" chimney, a large dome cover on top of the boiler, and his "Eastleigh" superheater.