Supplementary memorandum by Tom Winsor,
Rail Regulator (RI 21C)
This note has been produced by the Rail Regulator
for the Transport Sub-committee of the House of Commons. It is
a response to the Committee's request for a written note about
gauge corner cracking. The note states that gauge corner cracking
(GCC) is a particular type of rail fatigue defect that has been
increasingly observed in Britain during the last 10 years. Its
precise causes are still being researched, but rail fatigue has
been well understood for many years. Management and rail inspection
processes should be capable of containing the problem. The GCC
problem is not unique to the Railtrack network, but other railways
appear to have been more successful in managing it.
Rails are said to be defective when they have
to be removed from service prematurely ie before they would normally
be replaced. Rail defects generally appear as cracks and/or surface
damage, and they are typically caused by:
(a) internal flaws within the metal arising
from the manufacturing process;
(b) physical damage that occurs during installation
(eg rough handling);
(c ) physical damage that occurs in service
(eg surface damage arising from wheel spin); or
(d) metal fatigue resulting from the continual
passage of traffic.
Gauge corner cracking (GCC) is universally regarded
as a metal fatigue problem. It appears as surface cracking on
the running surface and inside edge (the running edge) of rails.
This is the part of the rail that is in contact with train wheels,
and is therefore subjected to repeated cycles of particularly
high stresses as traffic passes over the rail.
Nature of gauge corner cracking
There are several phases of development of GCC.
The GCC cracks appear as a series of transverse
cracks across the top of a rail (the running surface), and they
may extend down the inner vertical face of the rail. In the early
stage of development these cracks are fine, hairline fractures,
that may not even be immediately visible to the naked eye.
Continued loading under traffic causes these
cracks to grow. Crack growth initially occurs at shallow depth
(a few millimetres) below the rail surface. However, if left unchecked,
two things tend to happen. The near-surface cracking can lead
to significant spalling or "shelling", where the rail
surface actually begins to break up and pieces of metal come away,
leaving the rail surface pitted and corroded. This will be quite
visible, but the internal cracking will also grow deeper into
the "head" of the rail to form a transverse defect across
the rail section. If that growth is allowed to develop, the head
of the rail becomes less and less able to withstand the stresses
imposed by passing trains and it will eventually fracturethe
break usually being triggered by the passage of a train.
This type of rail defect is particularly serious.
In its report to the Regulator in October 2000, Transportation
Technology Center Inc. (TTCI) explained that:
"These types of defect are dangerous in
that they appear in groups, and a single break may lead to consequential
breaks and catastrophic rail failure."
These words were written before the Hatfield
disaster. The inquiry is yet to conclude and report its findings,
but it appears likely that it will determine that this failure
mechanism was indeed the cause of this particular accident.
Is gauge corner cracking a new phenomenon?
Gauge corner cracking cannot be described as
a new phenomenon. It is well known that fatigue defects do develop
in rails, although it is true to say that the precise form of
fatigue cracking known as GCC has been less well understood than
other forms of rail defects. However, it has been known about
in Britain for at least 10 years.
Railtrack has informed ORR that:
(a) the causes of gauge corner cracking are
not well understood; and
(b) the problem has recently been appearing
at a rapidly accelerating rate.
Whilst neither of these views is disputed, the
nature of fatigue cracking in rails and the need to properly manage
it, have been well understood by railway engineers for many years.
The following extracts from "British Railway TrackDesign,
Construction and Maintenance" (the track engineers' handbook,
published by the Permanent Way Institution in 1993) illustrate
Referring to rail fatigue defects known as "squats",
the book describes a form of crack development that is virtually
identical to GCC:
"These are fatigue cracks initiated at the
running surface by wheel/rail contact forces. They are visible
on the running surface...the crack propagates at a shallow angle
to the horizontal until it reaches 3 to 5 mm below the surface.
At this depth, it usually branches downwards to form a defect
which is very similar in appearance to a Tache Ovale. If left
in track, this would eventually produce a brittle fracture and
a complete transverse break of the rail"
"Developments in steel technology have reduced
and in some cases eliminated some traditional fatigue failure
types, whilst improved ultrasonic test methods and a better understanding
of fatigue mechanisms and fatigue life prediction, are helping
to contain the problem in service...Nonetheless the constant drive
for higher speeds and heavier axle loads will demand that a continued
effort be maintained to keep rail fatigue under control"
ORR's view, therefore, is that although the
precise manifestation of GCC may well be a more recently observed
phenomenon, the industry should have been quite capable of detecting
and understanding the consequences of such cracking in rails,
even if it did not fully understand the causes.
In fact, in its report to ORR in October 2000,
TTCI's conclusions indicated that Railtrack has not made sufficient
efforts to keep rail fatigue under control:
"An area of concern is automatic ultrasonic
inspection, where it is believed Railtrack has not yet adopted
best practice in technology.. It is also felt that inspection
frequencies and minimum actions for defect removal may not have
kept pace with increases in traffic. It is repeated that high
levels of breaks indicate either that defects are not being found
sufficiently by inspection, or that they are being left in track
too long." (Ref. P46, "Rail Failure Assessment for the
Office of the Rail Regulator", TTCI, October 2000)
Is the problem unique to Britain?
No. Many other railway systems report problems
with GCC. The TTCI report to the Regulator (October 2000) shows
that fatigue failures of rail (including GCC and "shelling"
of rail) are a major problem on some of the world's large passenger
railway systems. East Japan Railways report that more than 60
per cent of rail defects are fatigue failures, whilst both SNCF
(France) and DB (Germany) also list them amongst the most common
causes of defects that require rail to be replaced prematurely.
It is true that the causes of GCC are not yet
well understood, although many theories exist about the nature
of rail steel now being used, the criteria used for track and
vehicle wheel/suspension design, and the quality of rolling stock
maintenance. Railtrack has recently been drawing in leading academic
and technical expertise from around the world (including TTCI)
to study GCC. This group is due to report very shortly, and ORR
awaits its conclusions with much interest.
The possibility of comparison with overseas
railways' experience is very welcome, because it is likely to
pinpoint particular aspects of recent UK experience and practice
that may have contributed to the scale of GCC problems now being
suffered on Railtrack's network.
Nevertheless, ORR is not aware of any evidence
to suggest that other railways have suffered the same severity
of the GCC problem and service disruption, and must therefore
conclude that other railways have managed the problem more successfully
ORR is continuing to press Railtrack for its
detailed responses to, and plans for implementing, the recommendations
for the TTCI report. ORR sees this as an essential element of
achieving a long-term network recovery.