September 2015

ISSN 0013-2845

ETR

Railway Technology
Review

I N T E R N AT I O N A L E D I T I O N Planning ETCS
 Innovation in track components
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The next step -The Hybrid Machine

Sylomer® under-sleeper pads

Use of Sylomer® under-sleeper pads
to reduce ballast wear on a Scandinavian heavy-haul railway line
The heavy-haul railway line between Luleå and Narvik can conveniently be considered in two parts: theSwedish section (Malmbanan) and the Norwegian section (Ofotbanen). This line has been carrying
continuously increasing loads, and so the infrastructure manager decided in 2014 to insert under-sleeper pads as a means of improving the quality of the track geometry. This decision was influenced by the
positive experience that the Austrian Federal Railways (ÖBB) had had with the use of pads under concrete sleepers and also by projects such as Innotrack and RIVAS. This article contains a brief descriptionof the Iron Ore Line in Scandinavia and the challenges due to heavy train loads. It also explains how
under-sleeper pads work and describes the approaches tried out for switches and crossings.

1. OVERVIEW AND FACTS
Malmbanan is a railway line providing aroute for heavy freight trains in the north of
Scandinavia, linking the sea ports of Luleå
(Sweden) and Narvik (Norway). It is theworld’s most northerly electrified railway
line, has a total length of 473 km and is usedprimarily for the transport of iron ore andpellets from the mines in Kiruna und Malmberget to the ports [1].
The Swedish section in the south-easternpart of the region is state-owned, and theinfrastructure is managed by Trafikverket(the Swedish transport authority). The line’s
mixed traffic includes passenger and general-freight trains as well as the heavy freighttrains organised by LKAB (Luossavaara – Ki-

irunavaara Aktiebolag), the Swedish operator of the iron mines. These last-mentioned
trains can have up to 68 wagons with an axle
load of 30 t and they run over Malmbanan
at 60 km/h in a laden state and at 70 km/h ifempty. It takes two IORE locomotives to haul
the laden trains with a total mass of 8500 t
and a length of approximately 750 m.
Back in the 1960s, the railway line was designed for an axle load of 25 t, and then in
the 1990s upgrading work began for higher
loads. Since 2000, an axle load of 30 t has
been possible over the entire length of thethrough tracks between Narvik and Kiruna.
The output of iron ore has been increasing, leading in recent years to a continuous
growth in the tonnage transported. A further increase in traffic loads is expected be-

FIG. 1: Iron Ore Line between Luleå and Narvik [1]14ETR  |  INTERNATIONAL EDITION  |  2/2015

Mag.(FH) Harald Steger
Product management
Getzner Werkstoffe GmbH
harald.steger@getzner.com

Dr. Harald Loy
Research & development
Getzner Werkstoffe GmbH
harald.loy@getzner.com

tween now and 2018. Figures 3 and 4 compare the number of iron-ore trains and theannual tonnage transported in 2013 withthe expected loads in 2018 [1].

FIG. 2: IORE locomotive [1]www.eurailpress.de/etr

Sylomer® under-sleeper pads

As part of the renewal work on the sectionof the line between Gällivare and Koskullskulle the operator has replaced wooden
sleepers with concrete ones with pads.
Since 2014, some 20 000 concrete sleepers
with pads have been laid over a length ofapproximately 10 km. The concrete sleepers have been supplied by Strängbeton andequipped with pads of the type SLB2210G.

Iron-ore trains

Number oftrains per day
2013

Number oftrains per day
2018

Annual tonnage carried[million tonnes]
2013

Annual tonnage carried[million tonnes]
2018

Northern section(Kiruna – Narvik)

8

12

32

48

Southern section(Malmberget – Luleå)

3-4

4

14

18

FIG. 3: Expected increase in the tonnage carried on the line up until 2018 [1]2. CHALLENGES DUE TO HEAVY
TRAINS
There are limits to what ballasted track willwithstand, and these arise particularly withheavy trains. High axle loads and high freight
tonnages cause a disproportionate load on
the track. It is essential to use top-quality trackcomponents to guarantee the availability of
the lines by minimising the outlay necessary
for maintaining them. Having switched tohigh-grade rail steels and prestressed-concrete sleepers, the next-weakest link in thepermanent way is the ballast. It is common
railway knowledge that placing pads under
the sleepers contributes significantly to reducing ballast wear. This is primarily explainedby increasing the contact area between theunderside of the sleeper and the top layer ofballast. Synthetic pads have to be used for thispurpose, since they make it possible for ballaststones to become embedded suffering as little damage as possible. An ideal distributionof the load in the track, on the other hand,is achieved by increasing the elastic deflection curve, and for that purpose highly elastic
sleeper pads are optimal, since they do not
lose their permanent flexibility even if subjected to loads for decades. Permanently flexible
sleeper pads, which, thanks to a high degreeof plasticity, also embed the ballast better,
place tough demands on today’s material sciences. The very considerable loads caused by
movements of heavy freight trains are to beunderstood as a particularly tough challenge
in this respect. Finding ideal combinations ofelastic and plastic properties is not a contradiction, as is shown by current development
approaches for maximum loads.
Stresses on track components due toheavy traffic
The International Heavy Haul Association(IHHA) defines “heavy haul” as an operationthat meets at least two of these three criteria[2]:>> total train mass equal to or greater than
5000 tonnes>> annual traffic load equal to or greaterthan 20 million tonnes>> axle load of 25 tonnes and more
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FIG. 4: Expected increase in the tonnage carried on the line up until 2018 [1]FIG. 5:The line between
Gällivare and Koskullskulle equipped withwooden sleepers
before renewal

FIG. 6:
Concrete sleepers with
SLB2210G undersleeper pads

Sleeper pads
Whereas passenger traffic with high frequencies is to be found especially in theworld’s most densely populated conurba-

tions, typical lines carrying heavy, long-distance freight are to be found in other countries too, such as the USA, Canada, China, »ETR  |  INTERNATIONAL EDITION  |  2/2015

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Sylomer® under-sleeper pads

FIG. 7: Heavy freight train on Malmbanan

Australia and South Africa. Those countries
have already accumulated extensive experience in the design of sustainable railway
lines. A likely trend for the future seems tobe that the findings from these lines will also
influence other fast-growing infrastructures,such as the dedicated freight corridors thatsome countries are planning or for the networks of private mining companies, someof which are very extensive. The targeted
increase in the axle loads and the increasing
tonnages on heavy-haul lines place toughdemands on the railway infrastructure andits components. Leaving aside the breakage of tensioning clamps, it is the degradation of ballast, starting at the contact surface
between sleepers and ballast, that is one ofthe primary factors shortening the life of railway installations. Figure 8 shows by way of
example the problematical transitional zone
between an open track and a bridge structure. Such discontinuities in a track, which

ought generally to be constructed so as tobe as regular as possible, lead to a particularly intensive load effect with the fracturing and pulverisation of the ballast stones.
The formation of white patches is often aclear indicator that this is happening. The
consequence of ballast degradation is a continuous deterioration in the quality of the
track geometry. It is possible to counteract
such negative effects with top-grade undersleeper pads, often in combination with suitable rail pads.

3. UNDER-SLEEPER PADS MADE OFSYLOMER®
Under-sleeper pads constitute a yielding
springy layer underneath the sleepers andincrease the contact areas with the ballast
from 2-8 % (without pads) to more than 30%(with pads), depending on the boundary

conditions. It has already been demonstrated on many occasions that a higher contact
area leads to a better assimilation of the load
into the ballast and a reduction in the load
acting on the subgrade. Under-sleeper pads
prevent the breakage of ballast stones on
account of overloading, and the extent of
track settlement is reduced. There is also apositive impact on the formation of cavities.
In order to quantify the general long-term
effect of sleepers with pads, extensive investigations have been carried out in the track,
to measure the rate of track deterioration,
going back as far as 2001 [3]. The sections
studied provide impressive evidence of thepositive influences on the track superstructure for various boundary conditions. All the
track sections with under-sleeper pads show
a significantly reduced rate of deterioration.
Even in sections with only a thin layer of ballast, it proved possible at least to double thetamping interval. The higher the load on the
track, the more efficient the effect of thepads. Under-sleeper pads bring about an appreciable reduction in the maintenance outlay and the life-cycle costs of a railway line[4]. The quintessence is that the sections ofline thus equipped have a higher availability.
Today, under-sleeper pads are fairly consistently used in the following instances:>> as a standard component for concrete
sleepers for more conservative treatment of the ballast>> reduction in settlement for all designs ofballasted superstructure>> reduction in the formation of short-pitchand other corrugations in rails in tights
curves>> adjustment of the stiffness of the trackin sections with a reduced ballast height>> equipping of transitional sections between differing systems of track superstructure>> reduction in vibrations>> avoidance of ballast cavities, and

FIG. 8: Examples of typical problems encountered on heavy-haul railway lines throughout the world: destruction of ballast under concrete sleepers (left), fractured tensioning clamps (middle) and cracked sleepers (right) are mostly the result of excessively high dynamic loads

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www.eurailpress.de/etr

Sylomer® under-sleeper pads

Without under-sleeper pads

With under-sleeper pads

FIG. 10: Formation of cavities under concrete sleepers without pads. With sleeper pads, cavities
are avoided and load-transmission

Ballast contact pressure [N/mm²]
3.5
3.0
2.5
2.0
1.5
1.0

FIG. 9: Laying sleepers with pads

0.5
0.0

>> optimisation of geometrically inevitableballast differences in switches and crossings.
Not all materials are equally suitable formeeting up to these tasks. One material thathas been shown to stand out as extremely
robust and long-lived is Sylomer®, which ismade from special polyurethane (PUR). In
the manufacture of PUR a broad range ofproperties can be set by adjusting the mixing ratio of the reactive input materials tomatch the requirements profile. The principal components by volume are polyol andisocyanate, and the ratio between themhas an essential influence on the mechanical properties and the comparatively highultimate tensile strength of the finishedproducts. In developing PUR under-sleeper
pads with the aim of reducing ballast wear,
the decisive considerations are to achieve an
optimum mixing ratio and to choose the correct manufacturing process in order to finish
up with elastic and plastic properties in balance with one another. It is the outcome ofdevelopments that have taken several years
that we today have more advanced undersleeper pads available to us, which in particular compensate for the maximum loads
resulting from heavy freight trains and alsooffer an impressive technical performance,
as emerges from the following account.

4. CONTACT AREA AND BALLASTCOMPRESSION
It is thanks to the plasticity of the undersleeper pads that the top layer of ballast
is able to embed itself in the pad material.
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Unpadded (concrete sleeper)(Contact area < 5 %)

EVA

PU

5.9 %

27.8 %(Image shows 1.4 %)FIG. 11: Mean ballast contact pressures under the concrete block representing the sleeper.
Comparison between a concrete sleeper without pads, with EVA pads and with PUR pads. The
principal influence reducing the ballast contact pressure is the greater contact area

This is a very important, safety-relevant effect, which plays a significant role above allin resisting the lateral displacement of the
track. As research has shown, this resistanceis significantly higher when pads are present
than in the absence of this embedding possibility [5]. As far as safety is concerned, it has
been clearly shown that sleepers with elastic
pads have a positive influence on track stability [6].
The principal consequence of the pad flexibility is that the forces are transmitted more
uniformly to the ballast bed. Getzner has developed a methodology of its own for quantifying the contact area quickly and very accurately. A digital analysis of various studies
has determined that the effective area ofcontact with the ballast lies in the range of25 ­– 33 %.
Without the plastic properties of the padmaterial, the contact areas would generally
be much less.
Figure 11 compares the contact areas
without pads, with EVA pads and with PUR
pads (same stiffness). As would be expected,
this comparative view shows that the con-tact area of sleepers without pads is thelowest (1.4 % in the test after the cyclicalloading test and generally < 5 %), the EVA
pad material has a somewhat higher score(5.9 % in the test), but the PUR material has
the largest contact area (27.8 %, for the same
nominal bedding modulus of the materials).
The most important parameter influencing
contact pressures is the effective ballast contact area. The greater the contact area, themore uniform the load transmission, and theresulting contact pressure on the ballast in
the track is then correspondingly lower.
concrete sleeper without pads, with EVA
pads and with PUR pads. The principal influence reducing the ballast contact pressure is
the greater contact area.

5. CONTACT IN REAL TRACKSThe contact area was established on sleepers
removed from a test section of railway tracknear Furet in Sweden. Two different products
had been in use in the transitional zone between an open track and a bridge, namely
elastic sleeper pads of the type SLS1308G »ETR  |  INTERNATIONAL EDITION  |  2/2015

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Sylomer® under-sleeper pads

Contact area < 8%without under-sleeper pads

Contact area up to 30%
with under-sleeper pads

FIG. 13: Furet/Sweden: Abrasion and destruction of ballast under sleepers without pads

and synthetic ones of the type SLB2210G. In
September 2014, after remaining in service
for nine years in a track with an annual loadof around 12 – 15 million tonnes, individual
concrete sleepers with pads were removed.
The impressions made by the ballast in thesleeper pads were recorded using digital
analysis and the percentage of the contact
area relative to the total was computed. The
measurements made on the sleepers withpads in a railway track confirm the results
obtained from other measurements. The
contact area of sleepers with pads is aroundfour times higher than that of sleepers without pads. Figure 12 shows that the maximum contact area of sleepers without pads
is only 8 %, whereas those with pads achievea contact area of up to 30 %.
It was established that as a consequenceof the higher contact area the ballast wasless worn under the sleepers with pads.
The ballast under the sleepers without pads
showed clearly visible abrasion, indicating
ballast destruction (Fig. 13).

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6. IMPROVEMENT IN QUALITY OF
TRACK GEOMETRY
The positive effect of installing undersleeper pads made of Sylomer® has beenvalidated on various sections of test track indifferent countries [7]. For comparative purposes, individual sections were equippedwith further-developed PUR under-sleeper
pads. Test sections were also set up without under-sleeper pads in order to establish the differences in the tracks’ long-term
behaviour. Precision levelling was used torecord the tracks’ settlement behaviour. Figure 14 shows examples of mean settlement
curves. In this example, the trends were already clearly recognisable after 377 days.
The mean track settlement had reached avalue of 7.5 mm in the zone with pads. In
the track without sleeper pads, by contrast,
the track settlement had already reached amean value of 12.5 mm. That meant that thesettlement of the classical ballasted track
was more than 65 % greater than in the zone
with under-sleeper pads, which certainly reflects the experience from other locations

FIG. 12:
Furet/Sweden: Comparison of the contact areaof sleepers with andwithout pads

where such pads had been used – despite
the relatively short observation period.
The test sections of track in which sleeper
pads were installed displayed a much morehomogenous quality of track geometry,
which can even be recognised with the naked eye in certain circumstances, as illustrated in Fig. 15.
Generally speaking, stabilising the ballast
with under-sleeper pads results in higher resistances to lateral displacement of the track[8]. As a rule, the greater the contact surface,
the higher the achievable resistance to lateral displacement. It will be noted that, asa result of the improved quality of the trackgeometry, the formation of cavities can bealmost entirely prevented in track with under-sleeper pads [9].
For heavy-haul railway lines, further developments working towards “semi-plasticpads” may deliver an even greater benefit.

7. SLEEPER PADS IN SWITCHES AND
CROSSINGS IN THE SWEDISH RAILWAY NETWORKInternational experience, such as that of theAustrian Federal Railways (ÖBB), and positive results from the Innotrack project [10]led Trafikverket to decide to install undersleeper pads. In 2014, a framework contract was concluded for the delivery over a
two-year period of around 100 000 Getzner
under-sleeper pads for switches and crossings. Since 2014, approximately 40 000 such
sleepers have been laid in the Swedish railway network.
Given the complex geometries of switches
and crossings, the outlay in labour and financial terms for maintaining track there is verymuch higher than for plain track. It is especially the differences in the stiffness of the
track bed in the direction of travel, which isinevitably due to the design of the switchwww.eurailpress.de/etr

Sylomer® under-sleeper pads

FIG. 14: Mean settlement over time with and without sleeper pads after a test period of 377 days

es and crossings that lead to differences inthe settlement of the rails. This results inadditional stresses in the track superstructure when a train passes over it. Clear-cut
improvements in the stability of the trackgeometry and vibration abatement can beobtained here through the use of elastic
under-sleeper pads. The ballast is subjected
to less stress, and the tamping intervals canbe increased. The life-cycle costs of switches
and crossings can be very considerably reduced.
If the system of elastically supported

switches and crossings is to be installed ina targeted fashion in a ballasted track, it isvital to understand the demands made ofthe elasticity and also what happens whena train passes. This requirement led Getzner
Werkstoffe to develop a non-linear mathematical model based on the finite element
method (FEM). This model offers the possibility of placing the elastic under-sleeper padsof the Sylomer® or Sylodyn® type in the various zones within switches and crossings sothat the jumps in stiffness can be smoothed
out and thus significantly reduced.

HIGHSPEEDEND STOPSAFETY

8. CONCLUSIONS AND OUTLOOK
Since 2014, under-sleeper pads have been
installed on the heavy-haul railway line between Luleå in Sweden and Narvik in Norway. Trafikverket expects the use of thesepads to extend service life and improve theavailability of the line, thanks to a reduced
outlay on maintenance. Studies on the Furet
test section have already confirmed that
there is a marked reduction in ballast destruction if concrete sleepers are fitted withpads, given the increase in the contact sur- »HALL A _ STAND A34

A. RAWIE GmbH & Co. KG . Dornierstraße 11 . 49090 Osnabrück . Germany . Fon +49_541_912070 . info@rawie.de . www.rawie.de
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19Sylomer® under-sleeper pads

FIG. 15:The differing quality ofcomparable sectionsof track is obvious, with
under-sleeper pads (left)
and without them (right)

FIG. 16:
FEM model for optimisation the use ofpads on switches andcrossings

face. Furthermore, longitudinal-level measurements show that there is an improvement in the quality of the track geometry[11].
On the basis of these findings from test
tracks and their confirmation in international
experience, Trafikverket decided in favour ofconcrete sleepers with pads made of Sylomer®. Experimental reports from the AustrianFederal Railways (ÖBB), the Innotrack project
and RIVAS form the basis for the decision to
use them as a standard, with the aim of reducing ballast wear both on open track andin switches and crossings.
It is particularly on heavy-haul railway
lines such as Malmbanan that the track ballast constitutes the limiting factor. In connection with swapping wooden sleepers
for concrete ones, the demand arose for
additional elasticity to be provided under
the sleepers. Under-sleeper pads made ofSylomer® do indeed cause less wear and tearto the ballast.

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ETR  |  INTERNATIONAL EDITION  |  2/2015

Innovative developments in materials
show the possibility of combining elastic
and plastic properties in the track in an ideal
way. The results are both a better distribution of the loads in the track and a reduction in the contact pressures between thesleeper and the top layer of ballast. This, inturn, leads to a reduction in settlement in
combination with an improved quality of
track geometry. The benefits for the infrastructure manager lie in the reduced maintenance outlay and the higher availability of
the railway lines. The design of the classicalballasted superstructure benefits from lasting improvements through such innovative
developments. [3] Schilder, R.: Schwellenbesohlungen. Getzner Bahnfachtagung Schwarzenberg/Vorarlberg, Nov. 2007,
pp. 26-33[4] Veit, P.: Marschnig S.: Making a case for under sleeper
pads. International Railway Journal IRJ, Jan. 1, 2011,
pp. 27-29[5] Iliev, D.: Die horizontale Gleislagestabilität des Schotteroberbaus mit konventionellen und elastischbesohlten Schwellen. Dissertation, Technische Universität München, 2012
[6] Freudenstein, S.: Iliev, D.; Stahl, W.: Querverschiebe­
widerstandsmessungen an un- und besohlten Schwellen.
EI – Eisenbahningenieur, July 2013, pp. 20 – 26
[7] Loy, H.: Heim, M.: Measurements of Track Settlement
for IR – Trials of Sleepers with Under Sleeper Pads
(USP). Getzner Werkstoffe Report, SE03, Oct. 2013[8] Iliev, D.: Versuche mit elastisch besohlten SchwellenElastizität, Kontaktspannungen, Querverschiebewiderstand. Getzner Bahnfachtagung Schwarzenberg/
Vorarlberg, Oct. 2011[9] Auer, F.: Einfluss von elastischen Komponenten auf
das Gleisverhalten. ÖVG Tagung Salzburg, vol. 104,
pp. 53 – 55
[10] Innotrack - Innovative Track Systems: Recommendation of, and scientific basis for, optimisation of switches and crossings – part 1. www.innotrack.net, 2006[11] Berggren, E.: Classification of Track Conditions with
Respect to Vibration Emission Deliverable D2.1 – Part
3: Track parameters before and after mitigation ofvibration with under-sleeper-pads at Furet, Sweden.
www.rivas-project.eu, Report June 2012

References[1] Asplund, M.: Nordmark, T.: Gustafsson, P.: Comparisonof TOR Lubrication Systems on the iron ore line. Proceedings of IHHA conference Perth, Australia, June
2015, pp. 1 – 3[2] IHHA – International Heavy Haul Association: www.
ihha.net, visited on 27 May 2015

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