Shortly thereafter, two alternative options were explored, an electrified version of the InterCity 125 (known as the HST-E), and the Class 89 mixed-traffic locomotive; these were both intended to a peak service speed of 125 mph.

[11] Some officials within British Rail pushed for more demanding requirements for the future Intercity trainset; reportedly, BR's Director of Mechanical and Electrical Engineering (M&EE) was a strong proponent for increasing the top speed to 140 mph.

While BR's board had approved the ordering of a single Class 89 as a prototype, the Strategy Committee queried why the type had been favoured over a proposed 80-tonne Bo-Bo locomotive.

[11] Appraisals also determined that the Class 89 was comparatively inferior in financial terms, in part due to the InterCity 225's prospective compatibility with WCML traction, reducing its development costs.

Despite this, the HST-E effort was promptly aborted, while Brush Traction decided to de-prioritise work on the Class 89 after learning that it was unlikely to lead to volume production.

[11] At one point, it was envisaged that the InterCity 225 would be ubiquitous, even potentially having the capability built into it to operate over the southern third-rail network and within the Channel Tunnel; by mid 1984, such fanciful ideas were curtailed.

[11] A pre-qualification document was formalised, in which various requirements for the type were laid out; these included the need to perform mixed-traffic duties (day and night passenger, parcel and mail, and overnight heavy freight services), the haulage of both tilting and conventional rolling stock, a top speed of 225 km/h, a maximum cant deficiency of 9° without the provision of tilt equipment, and that the maximum unsprung mass could not exceed 1.8 tonnes.

[11] The prequalification document was issued to BREL, Brush Traction and the General Electric Company (GEC), as well as the French firm Alstom and Germany's Krauss Maffei.

[11] The tendering process was relatively complex, but a decisive move appeared to have been GEC's offer of a sub-contracting arrangement to BREL for the construction of the locomotive's mechanical elements.

Between 2000 and 2005, with support from GNER, Bombardier Transportation, HSBC Rail funded a complete rebuilding and refurbishment programme for both the Class 91 and Mark 4 coaches, called Project Mallard.

During their operation with Virgin Trains East Coast (VTEC) in 2016, there were plans to retain six to eight sets with shorter rakes for a new London to Edinburgh service, even with the Azuma takeover.

[18][citation needed] In June 2018, new operator London North Eastern Railway (LNER) inherited all 31 InterCity 225 sets from VTEC as part of the franchise.

[20][21] The withdrawals have gradually continued as more of the new Azuma trains entered service and at the beginning of 2020, it was planned that the final InterCity 225 sets would leave LNER's fleet by June 2020.

[24] With LNER returning the majority of their InterCity 225 sets to Eversholt Rail Group, it has been announced that other companies would be obtaining some of the Mark 4 carriages and Driving Van Trailers.

[26][27] From May 2020, Grand Central were planning to begin using Mark 4 carriages on its new London Euston to Blackpool North services with Class 90 locomotives hauling six-carriage sets.

[29] After losing the West Coast Partnership franchise, Virgin Rail Group had proposed using InterCity 225s on an open access service from London Euston to Liverpool.