For the heaviest of the boat trains between London and Dover, William Kirtley designed the class M, of which six were built by Neilson & Co. in 1877. Two further orders had to be placed in part with the company's own workshops in Longhedge. Four class M1 locomotives and eight class M2 locomotives followed, which were almost identical to the first. All locomotives received new boilers between 1898 and 1904. They were retired between 1911 and 1923.