Because of their location, the engine shed was initially known as Par. On 1 January 1879 a loop line was built to the Cornwall Railway station at Par after which the Cornwall Minerals Railway engine shed and adjacent station were known as St Blazey to avoid the confusion of two stations with the same name.
The Cornwall Minerals Railway was operated by the Great Western Railway from October 1877. A new, elevated coaling road and 45,000 gallon water tank was provided before 1908.
The Great Western Railway was nationalised into British Railways from 1 January 1948. The first diesel locomotive was allocated to St Blazey in November 1960. The last steam locomotive workings from the shed were on 28 April 1962.
The roundhouse has since been converted into industrial units but since April 1987 the adjacent wagon repair shed has been used to service diesel locomotives, local passenger trains, and wagons used for china clay traffic. The turntable has been retained to turn the preserved steam locomotives that still visit Cornwall on special main line workings. Goods traffic is still sometimes loaded at St Blazey in the marshalling yard adjacent to the depot.
Types that have been in regular use from the depot since the 1980s have been:
The yard was used to store several DMUs overnight for Wessex Trains for many years, and then First Great Western when this company took over the operation of local trains. This meant that several morning services started at Par railway station and evening ones terminated there.