Gravel fit for travel

RAILTRACK, A new technology on rail track monitoring has emerged that may bolster the travelling public's confidence in the safety of the troubled network.

It's an infrared scanning system that, when fitted beneath ordinary trains, can locate worn-out sections of the track bed, pinpointing areas where the stony ballast supporting the rails is weak, threatening to derail a train.

Ballast is needed to spread the forces that trains exert on the ground beneath the track. It must also allow water to drain freely.

But when ballast gets too old and crushed, things can go badly wrong. Ballast is built up from three layers of angular granite rocks, which measure 2-6 cm across. Gaps between the rocks allow rainwater to drain through.

But passing trains give the ballast a heavy pounding that gradually breaks the rocks down into smaller pieces. Fragments start to clog the gaps, so water can't drain through the ballast efficiently. It is difficult to predict when the ballast needs to be renewed, so it has to be inspected regularly. It's a laborious job.

Inspectors walk the track at night when traffic is light to examine it visually. They also test the ballast by digging pits and looking for damage.

Researchers at the University of Edinburgh have found an easier way. Using a ground-facing infrared camera mounted on a rail trolley, they have been able to pick up problem areas by measuring the temperature of the ballast during the night.

The camera is set to pick up temperature differences as small as 0.04 degree celsius. As they moved the trolley down the track they found that degraded ballast was measurably cooler than sections of track that are in good shape. Researchers say this is because the air gaps in sound ballast act as a heat insulator, so the granite chips hold the heat of the day for longer.

Fitting the infrared camera to trains could give maintenance workers a way of continuously monitoring ballast quality across the track network. — New Scientist

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