Developed by Sir Henry Fowler for the LMS and introduced in 1924 the new locomotives had a Belpaire firebox from new, wider side tanks, larger bunker and an extended smokebox.
16636, was part of a large batch of 50 locomotives built by the Hunslet Engine Company of Leeds, West Yorkshire, between 1927 and 1929 and originally numbered from 16625–16674.
The conversion to 5 ft 3 in (1,600 mm) gauge was simply done by reversing the wheels and renewing the tyres and crank pins.
Their frames were not altered at all and, possibly due to the light nature of their work, the engines do not seem to have suffered from widening the gauge.
Another distinctive feature, and uncommon on the NCC, was the provision of "dogs" around the circumference of the smokebox to keep the joint airtight.
Designated Class Y, the engines were at first used on local trains to Carrickfergus but this practice was discontinued when it was discovered that the bearings were inclined to run hot.
In late Spring 1960 the Ulster Transport Authority acquired two 0-6-4Ts from the former Sligo, Leitrim and Northern Counties Railway, which took over duties on Belfast docks.