Following trials with the prototype DP1 "Deltic" locomotive, which was built at the Dick, Kerr & Co works in Preston, an order was placed with English Electric for a production fleet of 22 units (reduced from the originally-planned 23), replacing more than twice that number (55) of Nigel Gresley Pacifics; as steam locomotives require substantial time to clean, fuel, and fire, such a reduction in the number of units could be undertaken without a corresponding reduction in working availability. The locomotives were purchased under a service contract, English Electric agreeing to maintain them, including their engines and generators, for a fixed price; this was British Rail's first such contract. Additional Deltic engines were produced to enable engines to be exchanged regularly for overhaul while keeping the locomotives in service.