BRITAIN'S rail network carried what is believed to be an all-time record of 46·5 billion passenger-km in the year to the end of March, up 7·6% on 2005-06.

With passenger-journeys expected to grow by a further 22·5% in the next seven years (RG 9.07 p540), the Department for Transport asked Network Rail to examine the case for introducing double-deck coaches as an alternative to lengthening trains on commuter routes into London, where the maximum length is currently 12 cars 20 m long or, less commonly, 10 of 23 m.

The NR study focused on four major routes between 100 and 130 km long: Waterloo to Southampton, Victoria and London Bridge to Brighton, Paddington to Oxford and Liverpool Street to Colchester and Ipswich. All have a substantial proportion of quadruple track, adding to the cost of infrastructure conversion if operational flexibility is to be maintained.

The UK loading gauge is notoriously tight, both in terms of height above rail and the width available alongside 915 mm high platforms which not only limit lower-deck seating to 2+2 at best, but restrict the distance between bogie centres because of centre-throw on curves. NR assumed that DD stock would require at a minimum conversion of the upper gauge to the UIC GB reference profile to provide at least 1?920 mm height in the saloons.

Surprisingly, perhaps, infrastructure costs to handle 16-car single-deck trains turned out to be higher than creating GB gauge for double-deckers. Accommodating 320 m trains from Brighton to both London termini is priced at between £540m and £995m, whereas GB gauge costs £460m to £800m. To this must be added around 30% to compensate train operators for disruption while the work is carried out.

Moving from 12 to 16-car trains provides 33% more capacity, although NR has doubts about filling seats at the rear of longer trains arriving at London termini where the exit is beyond the buffers. Estimated gains from DD trains range from 23% using 20 m cars with high-density 2+2 seating to only 4% where the single-decker has 3+2 seating throughout but the DD car has 3+2 upstairs and only 2+2 downstairs.