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INFORMED SOURCES October 2007

Double-deck trains – the definitive study?

Will a new report lay the spirit of Bulleid at rest? I doubt it.

Some concepts and technologies have risen from the grave more times than Christopher Lee in the Hammer Films Frankenstein series. Old favourites in this column include gas turbines for traction, magnetic levitation and moving block signalling on a mixed traffic main line.

Then there's double-deck trains. Politicians go abroad, ride double-deck stock in France or Germany and come back assuming that it's only antediluvian railway practices that stop their use to reduce overcrowding on London commuter services. My innovative engineering chums then come up with ingenious ways round the constrained loading gauge.

If you believe that the steel wheel on steel rail is the ground transport system for the 21 st century, then the obvious solution to lack of capacity is to built more railway – which is what they do in other countries. But when self generated boiling frogs are eating up your available funds, double-deck trains could be a cheap way to provide more seats.

So, as part of the work behind the White Paper ‘ Delivering a sustainable railway', DfT Rail commissioned a study by Network Rail into the potential for double-deck and longer trains. This was slipped onto the DfT Rail website without fanfare at the end of August.

Interface

Obviously this is an interface issue. So the study determined the potential capacity gains from double-deck 20m and 23m long vehicles, within constraints such as curvature, and also estimated the costs and practicality of raising the structure gauge to accommodate both the UIC ‘GB' and ‘GC' Reference Profile heights.

While fitting two acceptably high ceilings into the vertical loading gauge is the obvious challenge to the double-deck train, vehicle length is equally critical. This is because, even on the Continent, the lower saloon floor has to drop down between the bogies to give enough height for two saloons.

For longer vehicles the study took 26 m long French double-deck EMUs for a reference. As reported in this column, (August 2005) the Vehicle/System Interface Committee has been developing the new ‘Intercity' gauge which will accept 26m vehicles. The Intercity Express Programme(IEP) is expected to exploit the new gauge.

So why not 26m long double-deck commuter trains? Simply, because on curves there is throw over at the vehicle ends and centre. According to the study, this would limit the distance between bogie centres of a British 26m double-deck train to 17m, compared with 20m on the French equivalent.

Moving the bogie centres, essentially the points around which the bogies pivot, in towards the middle of the vehicles would reduce the length of the central passenger saloon area from 15.4 m on the French vehicle to 12.4 m on the British design. This is equivalent to losing two seating bays in the saloon.

Obviously some extra seats could be provided in the longer end sections., but even here curvature intrudes. If you look at a Mk 3 coach you will see that the ends, including the doors, are chamfered. To match the gauging profile of a 23 m long Mk 3, a 26m long vehicle may require a tapering profile over the last 2m at each end. This narrowing could require the removal of some seating to retain an adequate gangway width in the end saloon area..

Overall, the Study concludes that as a result of the bogie spacing requirement, a 26m double-deck vehicle would provide only 8 more seats than the 23m equivalent.

20 m optimum

Even 23 m long vehicles could be a challenge on some of the suburban routes south of the Thames , where the infrastructure has ‘historically' been optimised for 20m long vehicles. The effect of moving the bogie centres outward to improve lower deck seating capacity, would require what the study terms a ‘wholesale reconfiguration' of the railway particularly on the tighter curves.

This is particularly true at stations on curves. The report warns that the ‘wholesale re-alignment and re-construction' of Clapham Junction would be required to accommodate useful 23m long double-deck vehicles.

That said, on certain southern routes, longer vehicles could be deployed. The study considers London – Brighton the most likely while the South Eastern routes are the more restrictive.

Headroom

In addition to evaluating the application of double-deck vehicles on medium distance corridors subject to high growth, the study was also asked to consider the passenger acceptability of double-deck rolling stock. This meant determining a minimum acceptable ceiling height.

Two typical European vehicles gave saloon heights between 1930mm and 2000mm. A minimum value of 1920mm (6ft 5in) was chosen for the notional UK double-deck vehicle. This is similar to London Underground Tube stock but taller than the top deck of a double-deck bus.

While 6ft 5in might seem generous, one objective of the study was to ‘improve the passenger environment'

This includes accommodating a population that has been growing taller (as well as wider).

Once the saloon height was set, the height of the double-deck vehicle could be determined. This came to 4320mm which would require gauge enhancement to UIC GB clearance as a minimum. A UIC GC gauge vehicle was also evaluated, with a vehicle height of 4650mm and a cabin height of 2030mm, but not taken further.

Table 2 shows the study's comparisons between the notional 20m and 23m double-deck vehicles with existing rolling stock. It has to be said that the gains on existing high density stock seem low – particularly the 14.4% achieved with 20 m long vehicles. Note, too, that the largest gains are in the ‘low density' vehicles where a simpler solution to meeting additional demand would be to change to a high density configuration.

Chums with hands-on experience of double-deck design reckon that the Network Rail study erred in working back from Continental European vehicles rather than developing a design to meet UK operating conditions from first principles. And, of course, several designers have done just this

Table 2

Comparison of seating capacity

Source: Network Rail

Gauge

So much for the capacity of the vehicles. The study then analysed the costs of gauge enhancements to accommodate double-deck trains on three routes (Table 4). The report points out that the cost of disruption during this work would add around 30% to the contract price.

Overall, the report concludes that the Double-deck option could be implemented but would be costly for the claimed relatively small gains in seating capacity. There would also be considerable disruption to the route and services during the gauge enhancement work.

While the works required are described as ‘relatively straightforward in an engineering sense', each route contains one or two significant challenges. More work would be needed to confirm that the proposed solution was achievable.

Table 3 shows how the study broke down typical items of gauge enhancement work into five cost categories/ Table 5 gives a cost breakdown for introducing double-deck trains on the Brighton line.

Table 3

Table 4

London-Victoria to Brighton – infrastructure costs for UIC GB gauge double-deck trains

Works type A-E

Individual Items

Individual Items (Tunnels)

Supporting Items

Finally, Table 6 pulls together the gauge enhancement costs for London-Brighton plus two other potential routes.

Table 5

Summary of infrastructure work activity and comparison costs for the introduction of Double-deck vehicles

Source: Network Rail

Longer

In addition to double-deck vehicles the study also investigated the costs of extending train formations to 16 cars. While an extra four cars gives 33% more seats over the current 12 car, the study poses the question ‘would the extra seating be used or would individual behaviour of passengers still leave crush loading in the first four cars, or on the return loading at the exits of the country stations'?

According to the Study additional measures would be required to evenly distribute the loadings throughout the trains. Such management of passenger distribution would be needed to achieve reasonable dwell times.

Longer single deck trains are certainly more practical. For example the vehicles can be deployed on other routes across the network in times of perturbation or planned diversions. Such flexibility would also makes the vehicles more attractive to the rolling stock companies and cheaper to buy.

But, nothing is for nothing and the report notes that infrastructure modifications to accommodate 16 car trains in stations, depots and sidings would be costly. Infrastructure changes to accommodate 16 car trains on the London Victoria / London Bridge to Brighton route are estimated at £540-995million and for Liverpool St to Colchester and Ipswich at £500-920 million. Note that these are more than the cost of gauge enhancement for double deck trains.

Table 6

Comparison of train capacities double deck vs longer trains

LD – Low density

HD High Density

SD – Single Deck

DD – Double deck

Sources Network Rail

Which brings us to the final Table, comparing the capacity gains of double-deck and longer trains, all with 20m long vehicles. This suggests that a 12 car high density double-deck train has a similar seating capacity to a 16 car single deck unit.

But far from resolving the issue, I suspect that the Network Rail study will generate lively responses from the specialist vehicle designers. One such, with designs in service, reckons that Network Rail's four car high density double-deck is a bit short in the seating department at 324. He reckons 472 would be more like it.

Continues.........