Due to the unsteady running of the 0-4-2STs, the last of the order, 3660, was turned out in August 1889 as a 0-4-4T bogie side tank. The remainder of the class were altered to a similar layout over the following two years. 3660 was slightly different at this time, having a bogie that was six inches (152 mm) shorter and an overall wheelbase of 20 ft 4 in (6.20 m), rather than the 21 ft 4 in (6.50 m) of the converted locomotives.
The standard gauge 0-4-2Ts were converted to 0-4-4Ts in the same manner as the broad gauge locomotives, which were all eventually converted to standard gauge. By the end of 1892 the whole class of forty locomotives was to one standard design for the first time.
All forty 0-4-4T locomotives were rebuilt as 4-4-0 tender locomotives between 1899 and 1902. Twenty six locos retained their parallel domed boilers while fourteen received new Standard No 3 parallel domeless boilers (later replaced by the taper barrel version of that type).
Holcroft was of the opinion that the original inside and outside frames were modified whereas Le Fleming was of the opinion that new inside frames would have been required. However, as the modification also increased the coupled wheelbase from 7 ft 4 in to 8 ft 6 in it is unlikely that much of the original framing was re-used.
Two locomotives, 3521 and 3546, were transferred to the Cambrian Railways in 1921 to replace locomotives destroyed in the Abermule accident. They were allocated Cambrian numbers 82 and 95 respectively but these were never carried, the two locomotives being returned to the Great Western Railway with their original numbers when the two railways were grouped together in 1922.
On 13 April 1895,locomotives 3536 and 3537, hauling the Cornishman, exceeded the speed limit by 24 miles per hour (39 km/h), damaging the track. A passenger train hauled by 3521 and 3548 consequently derailed between Doublebois and Bodmin Road, Cornwall.
In 1898, locomotive 3542 derailed near Penryn, Cornwall whilst hauling a mail train. The locomotive rolled down an embankment, killing the driver. The accident was caused by a combination of the condition of the track and the locomotive oscillating. Following this the whole class was rebuilt as 4-4-0 tender locomotives.
^le Fleming, H. M. (October 1954). White, D.E., ed. The Locomotives of the Great Western Railway, part seven: Dean's Larger Tender Engines. Kenilworth: RCTS. p. G45. ISBN0 901115 18 5.
^ abTrevena, Arthur (1980). Trains in Trouble. Vol. 1. Redruth: Atlantic Books. pp. 10–11. ISBN0-906899-01-X.
Reed, P.J.T. (February 1953). White, D.E., ed. The Locomotives of the Great Western Railway, Part 2: Broad Gauge. Kenilworth: The Railway Correspondence and Travel Society. ISBN0-901115-32-0. OCLC650490992.
Casserley, H.C.; Johnston, S.C. (1966). Locomotives at the Grouping: Great Western Railway. Shepperton: Ian Allan Publishing. ISBN0-7110-0555-9.
Russell, J.H. (1975). A Pictorial Record of Great Western Engines, Volume 1. Oxford: Oxford Publishing Company. ISBN0-86093-398-9.
Waters, Laurence (1999). The Great Western Broad Gauge. Hersham: Ian Allan Publishing. ISBN0-7110-2634-3.