[5] According to Bob Tiller, GBRf's Engineering Strategy Director, while the Class 66 was affordable and considerably more reliable than its British-built counterparts, the locomotive was effectively unobtainable by the late 2010s due to its non-compliance with the latest National Technical Specification Notices.

[5] During the 2010s, it was observed that there were no in-production diesel locomotives available at the time that could deliver the desired performance while also fitting within the UK's restrictive loading gauge.

[5] Furthermore, designing one would necessitate the placing of a large and costly order that came with some risk, as a suitably compliant engine might not even fit the UK gauge at all, while an active decarbonisation strategy that sought to eliminate all diesel trains from the railway by 2040 would thus limit its lifespan considerably.

[5] Priority was given to those locomotives that were in good enough condition to still be moved by rail, necessitating the bogies, suspension, wheelsets, and brake equipment being functional.

[5] The rebuild scheme involves the replacement of the original Ruston-Paxman RK3 engine with the EMD 710 powerplant, while newer electronic control systems based on those present on the Class 66 were also installed.

[5] The body was then subject to various alterations to accommodate the revised air intakes, exhaust (of both the engine itself and the cooler group) and the modified external lighting clusters that conform to contemporary standards.

According to GBRf, elements of the donor locomotives, such as the bodies, wheelsets, bogie frames and suspension, traction motors, and fuel tanks were usually in good condition already or were easy to restore.

[5] The most prominent alteration of the rebuild is the adoption of the new engine, the 12 cylinder EMD 12N-710G3B-T2; it is identical to the powerplants installed on some of the later-built Class 66 locomotives (numbers 66752–66779).

[15][16][17] The driver's cabs have also been heavily modernised, the changes include the installation of new measures to reduce the levels of both noise and vibration that the occupants are exposed to.