There have been a number of variations during the design and manufacturing of the rolling stock, primarily due to the delays and changes to the programme for the electrification of the Great Western Main Line. The original intention had been to electrify the line to Cardiff. However, this was then changed to extend electrification to Swansea, a decision which was later reversed. Each of these changes implied an alteration to the type of train being delivered, as well as changes to the depots, which are also part of the project.

The largest, most time-consuming variations to the project were fundamental and all essential if the project was to respond to the delays in the provision of the electrified infrastructure. The subsequent commercial negotiations were complex and time-consuming. It should also be recognised that the complexity of the contracts, their variations, and the need to secure the lenders' approval inevitably meant that external advisors needed to be heavily involved. Nevertheless, both parties worked collaboratively to overcome these challenges. The Procuring Authority recognised that the primary objective had to be the achievement of fair, and properly established, negotiated and agreed pricing, as well as other operational and contractual amendments.