The 10 Class 23 locomotives were supplied in 1959 by the English Electric Company as a pilot scheme locomotive and were intended to work both suburban passengers services out of London Kings Cross as well as local pick up freights in that area north of London. They were powered by a Napier Deltic T9-29 two stroke engine with turbo-charging which produced 1100 bhp at a very high 1600 rpm.  It is difficult to justify the decision by the British Transport Commission to produce these locomotives given the low power, fast running expensive to maintain engine and excessive exhaust which would prove inconvenient in terminal stations and would prevent access to the Moorgate lines.  One possible scenario for this highly stressed engine which would provide a degree of credibility to the design is that the class was used to test the potential for a higher power main line Deltic providing more power than the class 55 units.  With two18 cylinder units generating around 2200 bhp a so called "Super Deltic"with 4000-4400 bhp would have been possible. 

In the event the locomotives proved to be an unmitigated disaster with multiple engine and ancillary failures which resulted in them being stored from 1961 prior to a refurbishment.  It would appear that the refurbished locomotives were more reliable but that fast running engine was always going to be more expensive than the slower running units from Sulzer and English Electric.  The locomotives were deemed to be non-standard when the  National Traction Plan was published in 1966. All locomotives were withdrawn from active service by 1971 although 1 survived at the Railway Technical Centre (RTC).  A project is currently underway to build a replica locomotive using a redundant Class 37 shell as a base and a recovered T9 engine. 

The locomotives produced a relatively high tractive effort of 47000lbf with a continuous rating of 30600lbf at 9.4 mph although some technical books indicate the continuous speed was 10.0 mph. The rail horsepower increased slightly as speed increased due to increased traction motor efficiency with a peak of 808 being reached at 50mph. This was followed by a slow decline with a low of 760 hp at the full rated speed of 75 mph as main generator off-loading starts to be significant. At speeds beyond 75mph the off-loading would be more noticeable however over-speeding appears to have been fairly rare with the locomotives.  The tractive effort curve shown is a derived curve as no official TE curves have been sourced to date however it has been derived from first principles using the traction motor characteristics and is likely to be very close to the actual curve.