Warship locomotives were divided into two batches: those built at BR's Swindon works were numbered in the series D800-D832 and D866-D870,[1] had a maximum tractive effort of 52,400 pounds-force (233,000 N) and eventually became British Rail Class 42.

Although these diesel engines were of German Maybach design, they were physically manufactured by Bristol Siddeley at their factory at Ansty, near Coventry, under licence.

In the mid-1960s, the WR decided upon maroon as its new house colour for mainline diesel locomotives, this going very much against standard schemes imposed by BR's overall management.

A handful of Maroon examples, including D809, D815 & D817 remained in traffic until 5 October 1971, and were finally withdrawn in this colour scheme, although by now wearing full yellow ends.

The D800s were originally intended for the Paddington–Birmingham Snow Hill route and tests proved that their lighter weight and higher power allowed them to run to a two-hour schedule with 368 tons in tow: one coach more than a Class 40 could manage.

These plans were put back when Paddington became the temporary London terminus of choice for Birmingham during the early 1960s, whilst BR's preferred route from Euston via Rugby was electrified.

Loads of greater than 370 tons would be required and the service remained steam-hauled until the advent of the more powerful Class 52 Western diesel-hydraulic locomotives.

The root cause of this worry was the effect of small-diameter powered wheels carrying far more weight per inch of tread than those of a steam locomotive.

These concerns arose particularly from experiences in the United States of America although the significant rail damage reported there was mostly caused by wheel sliding under braking with heavy trailing loads which were very unlikely to occur on BR.

Around this time, several D800 drivers began reporting uncomfortable lurching over points or on poorly maintained track at high speeds.

The "Bristolian" was decelerated by 5 minutes but an extra stop at Chippenham was inserted so that the drivers practically had no choice but to exceed 80 mph (130 km/h) to keep to time.

All speed running ceased after autumn 1960, when BR's timetabling methodology as a whole changed towards making all inter-city services more regular interval with standardised train formations and more intermediate stops.

The WR also took the opportunity to reduce its former rival's main line to single track for long stretches west of Salisbury and to sell off the "surplus" land – a move that is widely regretted today.

By 1969 only two services were booked for a pair of D800s, albeit losing a further 15 minutes off the schedule, and the timetable was largely recast into separate Torbay and Plymouth trains, instead of being split en route.

[6] Prejudice against hydraulic transmission among the heads of BR's engineering divisions led them to announce in 1967 that all the WR's diesel-hydraulics were non-standard and should be withdrawn as soon as possible.

It was, for example, physically impossible to accommodate a compressor as well as an exhauster, so the locomotives were unable to haul newer designs of air-braked coaching stock.

It was intended to equip D870 with ETH, and to provide the necessary additional power, the engines were planned to be uprated by the fitment of charge-air coolers.

Many withdrawn examples were hastily cannibalised for spare parts to keep the others going as stocks had been reduced in anticipation of a swifter end to D800 operation than was in the event possible.

D829 Magpie had gained celebrity status due to its appearance on the popular TV show of the same name and there was an attempt to preserve the engine.

[2][11] D818 became a "pet" of the employees of Swindon Works and was repainted from Rail Blue with full yellow ends back into its original green livery.

D832 was restored to full operational order using many of the parts from D818 and it is doubtful whether there would have been enough components available to restore both D818 (which was missing several major items) and D832 without an expensive search for compatible German items, although this scenario became reality in September 2001 when D832 was fitted with a Mekydro transmission which was sourced from an ex-DB locomotive in Germany.

For example, the correspondent claims to have seen D828 Magnificent throughout June 1971 and on 4 July 1971 being hauled dead by Class 22 D6326 with fire damage and presumed to be heading for withdrawal at Newton Abbot depot.

D831 Monarch worked the 16:05 Exeter St David's to Barnstaple service and 17:55 return on 6 October 1971, three days after its supposed withdrawal.

On that same day, the correspondent notes D814 in use to power a convoy of D832 and D812 to Plymouth Laira depot where D814 was finally withdrawn, with D812 remaining in traffic.

In 1980 a further livery was offered in the form of D823 Hermes in BR Maroon with small yellow warning panels, and the class appeared once more in the 1981 catalogue.