No. 18 Ailsa was built for the Jubilee Line extension of the London Underground in 1994 and used by the contractors who re-laid approximately 2/3 of the Port Erin line in conjunction with the I.R.I.S. project (a sewerage scheme which saw a main pipeline being laid beneath the railway in 2000-2002 resulting in shuttle services being operated, dubbed I Reckon It Stinks by its detractors). 

The locomotive was bought from the contractors upon completion of the work and named Ailsa in honour of Lord Ailsa, who had greatly contributed to the railway in the late 1960s. It was stated at the time that the loco would receive a Spring Green livery (sometimes known as “Ailsa Green” as it was standard livery at the time of his ownership) but it remained in plain white livery, until removal of some of its ballast weights which has resulted in a red oxide lower half with a white upper. 

The locomotive was originally built to work on the construction railway in the Channel Tunnel which accounts for the somewhat squat appearance, although the cab was extended upwards before its arrival on island. This also results in limited visibility for the driver which has seen it fitted with closed circuit television cameras to aid vision. However, this disability as well as the absence of a continuous train braking system (in this case vacuum brakes) and also a mechanically governed top speed of 16 miles per hour ensures that the locomotive is used largely on shunting and permanent way duties being unsuitable for passenger workings.