SOME THOUGHTS BY A LAYMAN ON RAILWAY ENGINEERING.
Amid all the argy-bargy now going on in the railway industry it may be
helpful if the following facts were underlined - or authoritatively refuted,
Railways in Britain were routed in the 19th century so as to need the
minimum of civil engineering compatible with the limited earth-moving
facilities of that time. This meant that rail tracks tended to follow river
valleys and/or existing roads designed for horse-drawn traffic, and to
circumvent topographical features. Routing was also influenced by vested
land-owning interests insisting on all sorts of detours.
This resulted in a railway system which served its purpose when 60.m.p.h.
was regarded as excessive speed and when much lighter loads were placed on
A fundamental feature of railways is that trains are steered round curves
by thrust from the outer rail which opposes the tendency of any moving
object to move in a straight line. This thrust is aided by ‘cant’ or
‘camber’- the outer rail being raised above the level of the inner. Gravity
can then be used to assist, or under a single set of conditions to
eliminate, thrust from the rail - a principle familiar to every cyclist.
Rail vehicles, having more wheels than a bicycle and travelling at a wide
variety of speeds, it follows that hardly ever will the gravitational force
provided by the ‘cant’ supply exactly the necessary steering force. Almost
always a lateral thrust from the rails will be needed to steer the train.
If the train is travelling at a speed requiring a steering force greater
than that provided by the ‘cant’, then the thrust must come from the outer
rail. If its speed is less, then it will need an outward thrust from the
inner rail to keep it on the track. These thrusts are transmitted by sliding
pressure of rail against wheel flanges, and is the almost unique example in
any industry of massive forces being transmitted through unlubricated
sliding surfaces - unless there are leaves on the line!
Transmission across unlubricated surfaces results not only in grinding
erosion, but in the setting up of vibrations, the frequencies of which
extend over a very wide range. This complicated frequency pattern is added
to the oscillation caused by the weight of the train, which itself is not
simple owing to varying speeds and wheel layouts.
All this adds up to the most favourable conditions for the onset of metal
fatigue, not only in rails, but in wheels. It is difficult, if not
impossible to forecast accurately the extent of metal fatigue with such wide
variables, and when it will become catastrophic. The latest dismissal of
cracks on the west-coast route as being ‘minor’ is ominous. Every
catastrophic failure of metal starts with a ‘minor’ crack.
It must be remembered too that the steering thrust varies not only with
the mass of the vehicle, its centre of mass and the ‘cant’ , but by the
SQUARE of its speed. A train travelling at 120.m.p.h. will therefore call
for four times the rail thrust of a 60.m.p.h. train, and its reactive thrust
on the rails will be four times as great.
No engineer in his right mind would today propose to run trains at
125.m.p.h. on tracks which incorporated such curves as Railtrack has
inherited, and for the inadequacy of which it is castigated.
We seem to be faced with the alternatives of reducing speeds considerably
or of straightening out tracks - with all that that implies.
As for ‘punctuality’, that is not a precise term. A train only runs on
time if its timetable is a practical possibility. For some time the writer
commuted to London on trains which should have arrived at Victoria station
at 8.25.a.m. They never did - thus making the timetable a figment of the
compiler’s imagination. Had the timetable been amended whereby the trains
were scheduled to arrive at 8.50.a.m. this would not only have been more
honest, but would have provided some confidence in the little men writing
out timetables. Unfortunately it would appear that compilers of timetables,
practical engineers and politicians are unable to get their heads together.
© The Estate of William John Green, 2004