Bi-modes and Western wire

WITH increasing numbers of the new bi-mode Class 800 trains entering service on the GWR, Keith Farr analyses whether these sleek units can match the performance of the venerable HST125.

AS IN the title, ‘bi-modes’ came first, arriving on the Great Western Railway before wiring was complete. Electrification had fallen behind schedule and exceeded budget; GWR therefore decided to make its fleet of InterCity Electric Trains (IET) totally bi-mode from the start so they would be up and running as soon as possible.

Otherwise, the rolling stock cascade programme would have been left in further disarray: GWR would have been unable to release InterCity ‘125’s to ScotRail, which in turn would have delayed the transfer of Class 170 DMUs to increase capacity elsewhere. Great Western’s intention to employ other displaced ‘125’s on semi-fast services in Devon and Cornwall would also have been jeopardised, as would the concomitant release of elderly ‘Sprinters’ and ‘Pacers’. The cascade sequence thus depended on the punctual introduction of IETs, electric or not; so bi-mode it had to be.

Class 800s are beginning to make their mark on the Cotswold line. Set No. 800023 leaves Moreton-in-Marsh on February 2 with the 09.54 GWR service from Great Malvern to London Paddington. FRASER PITHIE

The down side is that reliance on bi-mode gave the Secretary of State for Transport Chris Grayling an excuse for halting electrification at Cardiff instead of Swansea and, on the Bristol line, at Thingley Junction, Chippenham. Beyond those points, Mr Grayling seemed to think passengers would not notice their ‘electric’ train had metamorphosed into a diesel, despite lower speeds and subdued grumbling from underfloor engines in the centre vehicles.

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Time for a comparison: a Class 800/0 five–coach IET weighs 250½ tonnes empty, the drive-end vehicles powering when the train is operating as an electric, and the centre three, each with an MTU1600 R80L diesel engine, normally set at 750 brake horsepower, contributing up to 2,250bhp. Therefore, in diesel mode, a pair of five-coach 800s in multiple, weighing 501 tonnes, produces 4,500hp – the same as a 2+8 IC125 weighing 408 tonnes. In theory, at least, the power-to-weight ratios are 9.0hp/tonne for the diesel bi-mode against 11.1hp/tonne for the ‘125’.

Great Western Railway states the diesel generators can now produce 940hp each, in line with the intended Class 802 sets for West of England services and increasing the power-to-weight ratio to 11.3hp: but detailed analysis by members of the Railway Performance Society (RPS) suggests this maximum has not yet been utilised. At low speed, however, a bi-mode’s acceleration is set at 0.75 metre per second per second up to about 20mph, giving a faster immediate ‘get-away’ than that of a ‘125’. The acceleration rate then drops so, by the time the train reaches about 30mph, speed is increasing more slowly than with a ‘125’.

Read more in March’s issue of The RM – on sale now!

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