They were designed by Thomas Russell Crampton for the London, Chatham and Dover Railway (LCDR) as general purpose passenger locomotives.

Tenders were sought, and these were considered in July 1860, when orders were placed with several firms for what were to become the Echo, Tiger and Acis classes.

[5] In November 1875, William Kirtley (who had replaced Martley following the latter's death in 1874) allotted the class letter G.[6] The locomotives were then given the numbers 3–26.

Withdrawal began in January 1892, and by the time that the South Eastern and Chatham Railway (SECR) was formed at the start of 1899, nine remained in service.

[3] Lethe was renamed Sphynx in August 1862, because the railway staff found the name difficult to pronounce.