IT BEGAN with a tiny crack in the top of a rail near Hatfield on the main
line running north out of London. As trains hammered over the rail, the crack
slowly deepened, creating a fissure in the rail鈥檚 head. Last week, the weakened
rail fractured catastrophically beneath the London to Leeds express. It came off
the rails at 185 kilometres per hour, killing four passengers.
According to Railtrack, the company responsible for Britain鈥檚 railway lines,
the rail that broke and caused the crash was only five years old. The previous
rail at the site had lasted 13 years.
The broken rail was on the outside of a curve, and this exposed it to extra
stress. Railtrack says that the break seemed to be the result of 鈥渃orner gauge
肠谤补肠办颈苍驳鈥�.
Advertisement
In corner gauge cracking, the fracture begins on the inside of the outer rail
on a curve and spreads across the head of the rail, says David Ventry,
Railtrack鈥檚 senior track engineer. The crack gradually goes deeper into the head
of the rail at a shallow angle before turning to go down at right angles.
Typically, says Ventry, 鈥測ou get a series of these cracks about 10 millimetres
补辫补谤迟鈥�.
Corner gauge cracking is an increasing difficulty on Britain鈥檚 railways. 鈥淲e
don鈥檛 have a full understanding of the problem,鈥� says Ventry.
Paradoxically, part of the problem may be that modern trains cause less wear
on rails than older ones, Ventry says. This is because trains are better
balanced than they used to be, and don鈥檛 move from side to side as much. When
the older trains caused more wear, they may have worn down steel containing
minute cracks鈥� effectively removing them before they became
significant.
He also points out that corner gauge cracking isn鈥檛 as much of a problem in
Europe, where the fastest trains run on new lines with few curves. Britain has
increased the speed of its trains but kept the old lines. 鈥淲e have more curves,鈥�
says Ventry. And trains now take these curves faster than they used to.
The number of broken rails on Britain鈥檚 railways has risen in the past two
years. Last year there were 917鈥攕ome 20 per cent more than in the
mid-1990s. The rail regulator, who monitors Railtrack鈥檚 performance, has
repeatedly warned the company about the number of broken rails. In August, the
regulator called in consultants to study the reasons for the rise. The company鈥檚
report is expected next month.
Rod Smith, head of mechanical engineering at Imperial College, London, points
out that the weight of trains is an important factor. The damage trains cause to
the rails is related to the fourth or fifth power of axle weight. So an increase
in axle load of 10 per cent causes roughly 50 per cent more damage.
The train鈥檚 unsprung mass鈥攎ade up of parts such as the wheel and
bogies, where the suspension does nothing to absorb the shock of
impact鈥攁lso affects rails. 鈥淭here鈥檚 a lot of subtle science behind the
wearing of the rail,鈥� says Smith. He points out that broken rails are almost
unheard of in Japan. He puts this down to a combination of heavier rails,
lighter trains and sleepers that are placed closer together, giving the rail
more support.