As diesel units were being introduced throughout the country there was pressure on the LMR to build units for the St Pancras to Bedford services. On the 19th November 1956 Mr David Blee, General Manager of the LMR received a deputation which represented most of the authorities whose areas were served by these services. They were also accompanied by Dr C. Hill, Postmaster-General, Mr Christopher Soames, MP for Bedford and Parliamentary Under-Secretary, Air Ministry, and Mr Norman Cole, MP for South Bedfordshire. Mr Blee explained that the problem of the most suitable type of traction unit was being actively studied. "This study, together with the demand for diesel traction units in all parts of the country and the limitations imposed by the production capacity of railway and independent manufacturers workshops, means that diesel trains cannot be introduced in the immediate future on the St Pancras - Bedford line." He also stated that plans were being prepared to convert the goods lines between Harpenden and Bedford so that they could be used for passenger services, enabling improvements to be made in the suburban services. A small working party was to be set up, composed of representatives from British Railways and the local authorities, to confer regularly "with the object of meeting as far as was practical requests which might be put forward by the users of the service."
A study of developments in the population of the outer areas served by the route, and of peoples traveling habits was carried out before the new stock was designed and before the accelerated timings were decided. Construction finally began in the summer of 1958.
At a press conference in September 1958 Mr Blee stated that the first of the units was expected in April 1959, with deliveries expected to be completed by the end of that year, when a new regular interval suburban service from St Pancras was expected to start. This was planned to give a general cut of 10% in journey times between London and the stations covered. Outside the peak hours the pattern of services was to be: an hourly service between Bedford and St Pancras, calling at all stations between Bedford and Elstree; and hourly train between Luton and St Pancras calling at all stations, giving a half hourly service between Luton, Elstree and St Pancras; and an hourly train between Kentish Town and Barking. With augmentation during peak periods there would be 18 more trains each week day, and during peak periods 8-car trains were expected to be run. There was to be roughly 30min cuts to the journey time between St Pancras and Bedford, and about 20-25mins to Luton. The diesels would not provide a service to Moorgate, although these services would be speeded up. At a cost of £2.5 million, the scheme was the largest of its kind so far, but this cost included £400,000 for the provision of a maintenance depot at Cricklewood and the long awaited upgrading of the goods lines to passenger standards, together with improved signaling arrangements.
He stated that saloons were to be fluorescently lit, using the then new transistor equipment. Each 4-car set would have four 238hp Rolls Royce engines with hydraulic twin-disc torque converter, which had just been exhaustively tested on E50000 with very promising results. Mr Blee also announced that this scheme would be followed by a similar one, covering the Marylebone suburban services, requiring 15 4-car sets, and gave further details on the Blue Pullman project, for which the Midland sets were also expected to be based at Cricklewood. It was also thought possible that two diesel parcels vans would be included in the rolling stock for the St Pancras scheme.
More detailed plans from September 1959 showed that Bedford would have 44 instead of 31 trains other main-line expresses to and from St Pancras, Luton 88 instead of 50, St Albans 100 instead of 79, and Elstree 45 instead of 30. By semi-fast diesel train the running time for the 49 miles from St Pancras to Bedford would be 70 min, to Luton (31 miles) 43 min, St Albans 28 min, Elstree 20min. Trains stopping at all stations would take a little longer. DMU stabling sidings were installed at the north end of Bedford station.
The first set ran into St. Pancras on the 14th May 1959, for crew training purposes. As further units were delivered, the training was stepped up, and a problem emerged. Test runs to Moorgate had to be abandoned as their body lengths caused clearance problems in the tunnels.
Crewe training continued until the 28th September 1959 when they were introduced on preliminary services. There worked eight off-peak services a day, to the existing steam timings as all the parts of the project were not complete.