The Procuring Authority played an important role during the design and manufacturing phase of the project. At financial close, the Procuring Authority had developed a technical specification for the trains describing the output requirements, however the design specifications from the tender stage were limited. Targets had to be clarified, the specification had to be finalised into detailed design requirements, and these had to be reviewed. Input into all these stages was needed from both of the affected Train Operators, who sometimes had differing views. The Great Western Line has substantial demand from commuters who travel regularly into London, while the East Coast Line is used more for discretionary travel, and hence the specific needs (in terms of design and technical specifications) of the two lines can differ. During the design and manufacturing process, the Procuring Authority relied on their technical advisors, and considered it important to keep the same advisors throughout the entire process. In addition to providing the rolling stock, the Project Company was also responsible for constructing and refurbishing train depots.

The electrification of the Great Western Main Line was not part of the project. However, those works needed to be completed in order for the new electric trains to be tested and then used. When Network Rail did not deliver on time, the programme for manufacturing the rolling stock had to be altered to increase the number of bi-mode trains, and the design of the depots had to be updated to cater for the bi-mode trains' diesel engines. This is described in more detail under the heading "Key Events" below.