In order to begin replacing these older units, SWT issued in November 1996 a request for tenders to supply 30 new air-conditioned four-car EMUs, and in 1997 together with rolling stock lessor Porterbrook awarded to Alstom a £90 million contract for their delivery.

[11][16] Some railway industry commentators noted that SWT were going above and beyond the requirements of their franchise award in placing the order, and speculated that it could be a strategy to allay concerns about the fact that the same company—Stagecoach Group—owned both South West Trains and Porterbrook.

[17] All 30 units, numbered 458001–458030, were fitted out at the former Metro-Cammell works at Washwood Heath in Birmingham, which Alstom had acquired in 1989, with the bodyshells being built in Barcelona, Spain.

The trailer car was provided with a pantograph well and space for an alternating-current transformer, enabling conversion to 25 kV 50 Hz AC overhead-line operation if required at a later date.

[8] The protracted and difficult introduction of the Juniper fleet is credited with influencing SWT's decision in April 2001 to replace the rest of its slam-door units with an order of 785 vehicles from the competing Siemens Desiro family.

In a separate arrangement units 458001 and 458002 were transferred to Gatwick Express in September 2005 to act as a spare train for that operator, but they were returned to SWT in December 2006 without having been used.

[22] In addition to reliability problems, continued use of the Class 458 fleet was complicated by issues surrounding compliance with the Rail Vehicle Accessibility Regulations (RVAR) 1998.

[23][24] The exemption relating to the size of the internal passenger information displays expired on 31 July 2006—having already been extended from 30 September 2004—and the Department for Transport declined to provide any further extension.

[27] As a result of the award, and in keeping with commitments made in its tender, SWT implemented a new rolling-stock plan that called for the withdrawal of its 24 Class 442 units.

[26] There was some speculation from industry observers that this was made possible by Porterbrook significantly reducing the amount of money it charged SWT to lease the Junipers, given that there appeared to be no other operators interested in using the fleet.

[26] By the end of the following February, observers reported that only four Junipers had not yet been re-activated,[8] and that the general reliability of the fleet had improved so much that units now averaged 17,800 miles (28,600 km) between significant failures (defined as causing delays greater than five minutes).

[28] Between 2008 and 2010 the fleet was 'refreshed' at Bournemouth Traincare Depot, during which process the units were fitted with CCTV, new seats and tables in first class, and modified RVAR-compliant lavatories and doorway lights.

[36] The units of this "new" fleet would be used—either individually or in pairs of ten cars—to provide extra peak-time capacity on suburban services into Waterloo from the Hounslow, Weybridge (via Staines), and Windsor lines.

[37][38][39] The Department for Transport announced in December 2011 that it had accepted the proposal,[33] and an agreement between Porterbrook and South West Trains was signed in January 2012.

Twelve of the 16 driving vehicles were rebuilt by Wabtec subsidiary Brush Traction at its workshops in Loughborough, where their original driver's cabs were replaced with newly fabricated versions that included gangways and Voith automatic couplers of the same types used on Class 444 and 450 units.

[34] New Train Management System (TMS) software was developed to be compatible across the entire Class 458/5 fleet, simplifying maintenance and improving reliability.

SWT expected to receive the first two Class 458/5 units in May 2013—in time for them to enter service the following November—but due to delays in production the initial delivery did not take place until October.

[46][47] In March 2021 South Western Railway announced that it was abandoning its plans to use refurbished Class 442 units for long-distance services to and from London on the Portsmouth Direct line, citing continuing technical difficulties and future problems complying with accessibility regulations.