The "OP's" were generally referred to as "The Disaster Class" in England, and were defiantly NOT successful!...
Can you provide a reference for your statement? I believe you have the Deltic class confused with a different class of British diesel locomotives which under-performed.
The 3300hp Deltic locomotives had considerable success in high-speed passenger service. 22 units were built 1961-62 and remained in service until 1980-81, displaced by newer faster equipment.
https://en.wikipedia.org/wiki/British_Rail_Class_55
By the mid 1960s the Deltic hauled Flying Scotsman was achieving a 5-hour 55-minute time from King's Cross to Edinburgh with one stop at Newcastle[6] and this was the fastest ever timing, beating the pre war A4 hauled "Coronation" service's 6 hours, and without the priority over other traffic accorded to the earlier LNER train. As the East Coast Main Line was upgraded times dropped still further and by the mid 1970s the Flying Scotsman was reaching Edinburgh in 5 hours 30 minutes, still with one stop at Newcastle.[7]
The ultimate Deltic performance came on 2 February 1978 with a run on the 07:25 from Newcastle to King's Cross. In some respects the run was set up (the driver was about to retire) but the speeds were record breaking. The locomotive was 55 008 The Green Howards, it was hauling 10 coaches (343 tons gross) and on the leg from York to London it achieved a timing of 137 min 15 sec. This included various signal stops and other enforced speed reductions, the nett time is estimated at 115 min 45 sec, an average of 97 mph start to stop. The train achieved 113 mph on the flat between Darlington and York, 114 mph at Offord and 125 mph whilst descending Stoke bank.
https://en.wikipedia.org/wiki/East_Coast_Main_Line
Just after the Deltics were introduced, the first sections of the East Coast Main Line were upgraded to allow 100 miles per hour (160 km/h) running. The first length to be cleared for the new higher speed was a 17 miles (27 km) stretch between Peterborough and Grantham on 15 June 1965, the second was 12 miles (19 km) between Grantham and Newark.[5]
As the demand for higher speed intensified, the Deltics were superseded by the High Speed Train (HST), introduced between 1976 and 1981 ...
The Napier Deltic engine was originally designed for naval service. The triangular design was unique and provided a high power-to-weight ratio. The next link has an interesting animated graphic of the opposed pistons working three crankshafts.
https://en.wikipedia.org/wiki/Napier_Deltic
This one is on prominent display in the National Railway Museum at York, England. My photo 2007. I got to see and ride behind one in mainline operation in 1978. They sound different !