DP1, or Prototype Deltic is a demonstrator locomotive built by English Electric in 1955. This locomotive resulted in twenty-two similar locomotives being ordered by British Rail, which became their Class 55.

Service with British Rail
The locomotive first saw service on the London Midland Region of British Railways, but the intention soon became to electrify the major routes on that Region. On the Eastern Region, however, no diesel replacement of conventional design seemed to be available for Gresley's swift and powerful Pacifics, particularly the A4. Only the Deltic appeared to offer the power and speed required within the restraints of sensible axle load, and it was soon running on that Region.