Page "British Rail Class 55" Paragraph 2

Following trials with the prototype DP1 Deltic locomotive, 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 Gresley Pacifics.

A first, was that the locomotives were purchased under a service contract, English Electric agreeing to maintain them, including their engines and generators, for a fixed price.

Additional Deltic engines were produced to enable engines to be swapped out regularly for overhaul while keeping the locomotives in service.