Tender locomotives were not suitable for working this route, and nor were many of the existing tank engines which were not powerful enough.

As a stop-gap pending the preparation of a new design, the SER purchased three newly-built Metropolitan Railway B Class 4-4-0T locomotives from that railway in April 1880, which were used on SER services from Woolwich Arsenal via Blackfriars and Finsbury Park to Alexandra Palace.

The 1881–82 Ashford series were built for working through the Snow Hill tunnel and the tunnels of the East London Railway, and so were provided with condensers and short chimneys giving an overall height of 12 ft 6 in (3.81 m), one injector being omitted and a boiler feed pump fitted instead.

[7] The locomotives passed to the South Eastern and Chatham Railway in 1899 and 55 were rebuilt by Harry Wainwright to class Q1 between 1903 and 1919.

Thirty-two unrebuilt locomotives survived into Southern Railway ownership on 1 January 1923 with random numbers between 6 and 424.