The D5700 series locomotives built by Metropolitan Vickers were, along with many other types, a result of the 1955 Modernisation Plan for the railways where diesel locomotives were seen as a stop-gap between steam and electrification. A brief outline of the Modernisation Plan is given below.

January 1955 saw Sir Brian Robertson, Chairman of the British Transport Commission, announce plans for the complete modernisation of Britain's railway network which was to be backed by the Government. The stated aim was 'to exploit the great natural advantages of railways as transporters of passengers and goods and to revolutionise the character of the services provided for both'; to transform the quality and cost of passenger services so as to attract a larger share of traffic offering, and similarly 'to recover the ground that has been lost to other forms of transport over the past thirty years' in freight traffic', the latter statement could well have been written by EWS today, but I digress...

Observers noted that it would be too little too late, bearing in mind the advance of road transport (private and commercial) and the aspirations of the airlines. The main item of concern to the enthusiast was the announced intention to phase out steam as soon as was practicable. This was indeed a strange turn of events as the Standard steam locomotives were still in full production (and would remain so for a further 5 years!) and were expected to be in service well into the 1980's!

Expenditure of £1,240 million spread over 15 years was envisaged, (it eventually topped £1,600 million!) but it was expected that half this sum alone was needed just to keep the railways in their current condition. The money was to be spent on stations, goods yards, track, bridges, signalling, communications, motive power, coaches and wagons. The following months saw further details released and it was clear that the BTC envisaged a very different railway to the one prevalent at that time. It could be argued that much of the money was to be wasted on such schemes as new hump marshalling yards at Carlisle and Tinsley, etc, as investment was spent on traffic flows that were in terminal decline. The same could have been said about the investment in modern diesel traction.

Before the final decisions were taken regarding bulk delivery of the anticipated 2,500 locomotives required, a pilot scheme of about 170 locomotives was to be brought into operation for assessment. These locos were to appear from many different locomotive builders (many of whom with no experience of main line locomotive construction) and were hastily cobbled together for evaluation. Upto 5000 diesel multiple units were also planned to be built during this period, but do not come under the scope of this article.

The three main power categories for the locomotives were :-
Type 'A' (600-800hp)
Type 'B' (1000-1250hp)
Type 'C' (2000hp and over)

This was later revised in 1957 as follows :-

To give a comparison of performance. A single Type 3 was deemed to be an equivalent to a Stanier 'Black 5' steam locomotive.

The well meaning idea that these pilot batches of locomotives would be evaluated and the better performers ordered in bulk, but this idea was soon scrapped (like most of the ensuing locos!), such was the desperation of the British Railways Board for cost efficient diesel power.

In May 1957, 230 locomotives were on order without a single example in service. A reappraisal of the Modernisation Plan was initiated in late 1958 and its implications were that the scheme should be accelerated without delay to gain the promised benefits as soon as possible. This meant the virtual abandonement of the Pilot Scheme and large orders were placed without a single prototype running. In effect, larger batches of locomotives were ordered straight from the drawing board! The end of 1963 was set as a target date for delivery of 2300 diesel locomotives. Any concept of standardisation was a non-starter which was one objective of the Pilot Scheme orders - in order to find the best for standardisation, each manufacturer had been granted a free hand to demonstrate their ideas and equipment, leading to inevitable diversity of designs both mechanically and technically. There were to be 14 different types of locomotive!

Thankfully, in June 1957, the first Pilot Scheme diesel locomotive was handed over to the BTC - the 1000hp D8000 from the English Electric Company's works at Vulcan Foundry, Newton-le-Willows, Lancashire - birthplace of "Deltic" two years earlier, the 3300hp privately sponsored locomotive - the aesthetic appearance of which was much criticised by journalists and the public alike. One outraged observer was reputed to write to a national newspaper saying it was like "a Zulu in full warpaint", a reference to the striking livery of bright blue and gold stripes . It was, however, a success in service, leading to the order of 22 production examples for the East Coast Main Line.

The second type of locomotive to be delivered under the Modernisation Plan was D5500 from Brush Traction Ltd's Kestrel Works at Loughborough, Leicestershire, rated at 1250hp.

Despite the success of the D8000 series, the Brush Type 2s were to be ominously beset by engine problems, leading to a complete re-engining programme in due course. Other types would not be so lucky.

As discussed in the technical details, British Railways invited the Co-Bo's candidacy in the Piolt Scheme chiefly to evaluate a two-stroke diesel engine against the four stroke models adopted for all the other types.