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Debate Trackbed technology

Revelations that even recently-laid rail is suffering from breaks, head checking and guage corner cracking have cast doubt on the ability of current rail technology to provide service and reliability. The basic technology still used on most of the UK's tracks dates back to the 1850s. This week we ask: Is it time to abandon traditional trackbed technology?

Yes

Britain's railways infrastructure is lagging behind many of our competitors. In particular the lack of any truly high speed rail stands out. Where it is now routine to travel at 250-300km/hr in Japan and some European countries, we struggle to reach 200km/hr. In France, it has been proven that high speed is not incompatible with ballasted track, but at a massive maintenance cost. Japan, arguably the world leader in rail operating efficiency, has over 25 years of experience of rigid concrete track bed, and in Europe Germany is now leading the way.

The relatively high initial cost of concrete slabtrack, some 30-50% more than ballasted construction, is offset by long low maintenance life and a safer, smoother ride. There are additional profits to be shared as unplanned delays are minimised and passenger satisfaction increases.

Extremely accurate track geometry can be achieved with concrete slabtrack systems, which produces an ultra smooth ride for passengers (no more spilled cups of coffee! ) and less wear and tear on train suspension systems. Most importantly, in the wake of the Hatfield disaster, rail breaks are rare and much less likely to be catastrophic if they happen on concrete slabtrack. Slipform paving techniques mean train restraint barriers can be paved as an integral part of the slabrack system.

The slab is also lighter and shallower than the equivalent ballast bed, giving designers significant engineering benefits by reducing deadweight over bridges and viaducts, and in saving headroom in tunnels. Other opportunities for slabtrack are in loop lines, crossovers and heavy freight lines. With safety so high on the agenda, slabtrack could be used in high risk areas such as curves, or where rail breaks have been a problem.

The new Dutch High Speed Line South linking Amsterdam to Brussels is believed to include concrete design options which virtually eliminate the rail break problem, a fine example of engineering for maximum safety and reliability.

Concrete slabtrack systems deliver the ultimate in reliability.

In 1999 on the Tokaido Shinkansen in Japan 96.1% of trains on the Tokyo to Osaka route arrived exactly on time and only 0.8% were 10 minutes or more late!

For a step change in safety, speed, comfort, sustainability and reliability, the UK must look to concrete slabtrak.

No

At first glance this may appear the case, but the matter is not that simple.

Railway track comprising continuous rails on concrete sleepers has a good maintenance record, is adaptable to changes in geometry and is satisfactory for use on most of the railway system.

The purpose of the railway is to provide transport efficiently to a published timetable and to disrupt this to the minimum with engineering work.

Railway track undergoes aggressive loadings, is subjected to repetitive flexure, has a finite life span and eventually needs complete replacement.

In the event of derailments sleepered track can be speedily replaced and restored to traffic.

I would be interested to see how an insitu slab alternative would meet these criteria within the live railway and how the promoters of the system would envisage its installation and renewal under possession.

The rail is effectively a continuous beam and wheel loads induce inescapable uplifts of the sleepers.

Tests on simple slabs in the short term have shown pull out of fixings, spalling of concrete at outer edges on curves around these, and flexural cross cracking.

Faults like these would indicate to me that a whole range of maintenance problems would arise with these slabs, some of which are likely to be intractable and would no doubt be present throughout the whole life of the slab.

Track slab details incorporating the use of discrete cast iron base plates and spring loaded fixings have however been used in tunnels and underbridges where track geometry dictates. I am not aware that these are intended for any other purpose.

I believe the use of track slabs will not reduce the number of broken rails. The stiffness of the slab could well increase the impact factor at the rail to a much higher level and exacerbate the problem together with that of gauge corner cracking (GCC).

In my view, the pattern of broken rails should be examined in conjunction with GCC events, diagramming of trains, wheel loads and the condition of wheel profiles, to formulate a solution to the current problems, rather than moving to slab track.

The facts

Railtrack is responsible for 31,800km of tracks.

Some 66% of these are based on concrete sleepers, 32% on timber and 2% on steel, although the use of steel sleepers is said to have increased recently.

In 1998-99 there were 952 broken rails on the network. This fell to 919 in the following 12 months.

Figures for 2000-2001 are still unavailable, but Railtrack is predicting a further fall to below 750.

In an average year there will be 20 'track-related' derailments in the UK. Trough shaped concrete slabtrack systems are claimed to be capable of restraining derailed rolling stock and preventing the type of secondary collision that occurred in the Great Heck disaster.

Railtrack is still considering setting up a full scale trial of concrete slabtracks.

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