Charles Collett became the Chief Mechanical Engineer in 1921. Almost straight away he had to take on all the locomotives of myriad types from the railways absorbed in 1922 and 1923. Many of these were 'Swindonised', that is they were rebuilt using standard GWR parts. He also set about designing many new types to replace the older examples. Many of the most familiar GWR tank locomotive classes were designed during this period: the 1400 class for small branch lines and auto trains; the 4575 class (a development of the 4500 class with larger tanks) and the large 6100 class 2-6-2Ts; the massive 7200 class of rebuilt 4200 class 2-8-2Ts; and the iconic pannier tanks of the 5700 class, the first of which appeared in 1929.