The rolling stock for the Great Western Main Line was originally due to be split between 29x five-car bi-modes, 15x eight-car bi-modes and 13x eight-car electric trains. Bi-mode trains are electric trains that are equipped with underfloor diesel generators to provide propulsion where lines are not electrified. There had never been plans to electrify all the lines on which intercity trains operated on the Great Western network. Having part of the fleet bi-mode allowed continued operation onto the non-electrified routes and also a degree of operational flexibility, in particular to use non-electrified diversionary routes during engineering work or disruption. There were contractual commitments to provide the electrified track for both testing as well as for operations.

In early 2015, it became apparent to both the Procuring Authority and Project Company that the planned electrification of the Great Western Network, necessary to support both pre-commissioning and testing activities as well as the eventual operational deployment of the primarily electric IEP fleet, was not going to be delivered according to Network Rail's original timetable.

To mitigate this forecasted delay and its associated implications, the Project Company and Procuring Authority worked to develop a number of contractual variations that: (i) converted the electric-only IEP trains into bi-mode IEP trains able to run without overhead wires, (ii) made the necessary modifications to the depots to accommodate and service diesel trains and (iii) addressed the commercial consequences of the delay in the provision of the necessary testing infrastructure and the resultant delay to the original entry into service date.

The Procuring Authority and Project Company were able to deal with this challenge successfully due to the strength of the relationship between the two parties, and also the commitment of the Project Company, together with its manufacturing contractor, Hitachi, to deliver the rolling stock with as little delay as possible. A commitment to finding a practical way to overcome challenges was seen as vital by all parties and, again, working closely with Hitachi, a revised delivery schedule was agreed, and the costs of delay were mitigated. There have been no formal disputes between the parties.