This was a major change from BR's existing traction policy, drawn up immediately after nationalisation, which had been based on perpetuating steam locomotives: existing locomotive designs from the Big Four continued to be built post-nationalisation, and then BR designed a new series of standard locomotive classes. Entering service in 1951, the Standards were intended to have a service life of 30 years and would be superseded by a rolling programme of electrification. In its early years BR largely halted the work done by the Big Four experimenting with diesel traction – completing pilot orders for prototypes such as those from the Southern Railway which would become BR's Class D16/2 – but not perpetuating them. The exception was the continuation of the introduction of a series of diesel-powered shunting locomotives such as what would be the British Rail Class 08 and its variants, which entered service from 1951. The Modernisation Plan overturned this arrangement, even though many of the orders for the Standard steam locomotives were years from being fulfilled. Steam traction would now be replaced by diesels within a decade while the long-term electrification programme, while retained, would be slowed and scaled down. It was hoped that the rapid switch to diesel traction would deliver similar operational advantages and cost savings as electrification but at a faster pace and with much lower upfront capital costs. This committed many of the later-built Standard steam locomotives to be withdrawn having served only a third (or even less) of their intended service life.