The boiler was higher pitched than the original to allow the firebox and ash pan to clear the rear driving wheel axle and an extension of 24.75 inches (630 mm) to the smokebox saddle was needed to offer support.
Although the effect was less striking than that of the similarly rebuilt Class B3, the modified appearance was such that these locomotives would be included among those that gained the nickname of "Whippet" because of a perceived resemblance to the racing dog.
The majority of the Class A1 engines were coupled to what was known as the "Standard" tender which could carry 6 long tons (6.1 t) of coal and 2,090 imperial gallons (9,500 L; 2,510 US gal) of water.
The initials "NCC" in shaded serif gold capital letters were placed centrally on the tender sides.
Number plates were brass with raised digits and edge; they were carried on the lower cab sides with another placed centrally on the back of the tender tank.
The UTA roundel, 14 inches (356 mm) in diameter, with "Ulster Transport" in orange block capitals, lined in red, surrounding a white shield bearing the red hand of Ulster, all on a mid-green background, was placed in the middle of the tender sides.