Optimising the track bedding stiffness and settlement behaviour at insulated rail joints
Besides turnouts, insulated rail joints are the most frequent cause of track failure. They disturb track
homogeneity and cause a local variation in track bedding stiffness, leading to an increase in dynamicloading. Local variations in track bedding stiffness due to turnouts, insulated rail joints, as well as othernecessary devices in the track, lead to an increase in maintenance demand and inherent costs. By im­
plementing under-sleeper pads, the track bedding stiffness and settlement behaviour at insulated rail
joints can be optimised, which has been confirmed by measurements conducted in the USA and China.
Insulated rail joints with an optimised track stiffness yield promising maintenance-related and economic
benefits, as also alluded to in this article.
By: Dipl.-Ing. Martin Quirchmair, Development Engineer - Railway Division, Getzner Werkstoffe GmbH, Bürs, Austria;
Assoc.-Prof. Dipl.-Ing. Dr.techn. Stefan Marschnig, Associate Professor, Institute for Railway Engineering and Transport
Economy, TU Graz, Austria; Dipl.-Ing. Michael Fellinger, University Project Assistant, Institute for Railway Engineering andTransport Economy, TU Graz, Austria; Dr. Harald Loy, Head of R&D - Railway Division, Getzner Werkstoffe GmbH, Bürs &Postdoc Researcher, Intelligent Transport Systems, University of Innsbruck, Austria.

INSULATED RAIL JOINTS: THEIRIMPACT ON TRACK HOMOGENEITY
Signalling technology is an integral part of modern railway
networks. Many electrical signals are still transmitted via therail, with the track vacancy detection system being one of themost important safety technologies (Fig. 1).

SECTION FREE

TransmitterFig. 2: Schematic drawing of an insulated rail joint

Fig. 1: Principle of trackvacancy detection

In order to guarantee the function of track vacancy detection
systems, defined sections of track must be electrically separated
from one another, which is effected by the adoption of insulatedrail joints (Fig. 2). Typically, these are arranged at 1-3 km
intervals and can also be found at the beginning and the end ofturnouts. The use of insulated rail joints has great benefits interms of signalling, but this is at the expense of track homo­geneity, as their static and dynamic behaviour differs from thatof the adjoining track sections.
The ideal track is characterised by a uniform static anddynamic behaviour throughout the entire railway network.
The change in track bedding stiffness at insulated rail joints,
however, leads to a local increase in dynamic forces, which
results in an accelerated wear of the components of theinsulated rail joints, as well as a deterioration in track geometry
quality in their immediate vicinity. Typical damage that has been
observed includes increased metal flow at the rail ends (shortcircuit), end-post battering, and ballast quality deterioration.
The latter causes an increase in rail deflection, which leads
to higher mechanical stresses in the insulated rail joint andexacerbates the situation further. All this results in a rise inmaintenance demand and inherent costs, both regarding thetrack and the insulated rail joint itself.
The service life of insulated rail joints depends on the typeand intensity of traffic borne. Australian studies have observed
a service life of only 50 million load tons for freight traffic,
whereas in the USA the figure is around 200 million loadtons. This corresponds to an effective service life of only 12-18
months, leading to additional costs of some $10,000 per mileeach year.
Rail Engineering International Edition 2020 Number 4

In Europe, Network Rail (UK) is investing flO million overa two-year period due to defective insulated rail joints [1].
Insulated rail joints have also been identified as a problem area
by Austrian Federal Railways (ÖBB). In Austria, they are themost frequent cause of track failure, accounting for approx.
40%, when not taking turnouts into consideration (see alsoFig. 3) [2].
Track Malfunctions KAT.1 2016 (without Turnout)
Source: ÖBB Infra
FalseAlarm
12%

Alignment Errors
10%

Broken Rails
8%

Fig. 3: Kat. 1 trackfaults (excluding tumouts) in 2016
(Kat.] corresponds to traffic hindrances, i.e.
full closure or speed restrictions) [2J

Maintenance of jointed track - a costly matter
Concerning track maintenance, rail joints are a factor that mustnot be neglected. Particularly in the case of jointed tracks (inAustria limited to track sections featuring curves with very tightradii; today they are only implemented if the rails cannot be
continuously welded due to regulations), "joint upkeep" re­presents a considerable cost factor. A jointed track in a tightcurve has an around five times higher life-cycle cost (LCC) thanstraight continuous welded rail (CWR) track [3], due to its
higher maintenance demand and shorter service life (Fig. 4).
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