The first three high capacity conversions to Class 450 stock were introduced to service on 21st January 2008. The first two in traffic were 450548 & 450555 working:
Off Waterloo :
07:28, 09:58, 12:28, 14:58, 17:28, 19:58 & 22:28
Off Windsor :
06:23, 08:53, 11:23, 13:53, 16:23, 18:53 & 21:23
Unfortunately the third unit, 450554, was failed before entering service.

These conversions are part of 's strategy to meet their new franchise's requirement for additional passenger capacity on the “Windsor Lines” from Waterloo to Windsor, Weybridge via Chertsey and on the Hounslow Loop.

	450554, with 450548 leading, bringing up the rear of the 11:03 departure from Wandsworth Town on a Waterloo to Woking via Hounslow service on Sunday 3rd February 2008 photograph by James Dixon
	Interior of what was formerly the first class section but following conversion is now a standard saloon in what has become a trailer open standard lavatory brake vehicle. photograph courtesy of Colin J. Marsden

Conversion of 28 Class 450/0 Desiro sets is being done by Siemens at Bournemouth depot. The specific sets to be converted to high capacity are 450043 to 450070, to be renumbered into the new 450/5 sub-class by adding 500 to their original number. The modification work takes around 15 days per set to complete and four sets will be modified at the same time. The modifications involve the following:-

• In the former TOCLB vehicle: removal of first class seating and replaced with standard class seating in 2+2 and 2+3 style, removal of two standard class seats from the standrd class saloon, installation of extra handrails and removal of sliding doors previously separating the first class area thus providing room for 40 extra standing passengers. In the former first class saloon under luggage-rack lighting and lap top/telephone power points have been retained but the curtains have been removed
• In the Trailer Open Standard Disabled Toilet (TOSLW) vehicle: removal of two seats from the toilet end; provision of extra handrails and new litter bins between seat backs
• In the two Driving (DMOS1/2) cars: removal of six seats by the door openings; provision of extra handrails and three new litterbins.
• Throughout the train all of the vertical litterbins originally installed by the doorways have been removed
• Replacement of the scratch resistant surface on the windows; application of a graffiti resistant finish to the walls of the toilet compartment and gangways areas

The modifications give a net loss of 16 seats, but an increase in standing capacity. If confined to the Windsor lines it is claimed this should result in better conditions for standing passengers in the peaks. The recent DfT announcement regarding new rolling stock has indicated that additional Desiro vehicles may be provided. This could potentially be to lengthen the 450/5 units by 1 extra vehicle, creating in effect the five car inner suburban 450/2 units that were originally destined for SWT before being redirected to Silverlink County (now ) as 4 car units.

450554 again, this time at Vauxhall on 8th February 2008 whilst working a Hounslow loop service. photograph by Ian Docwra