Question

There are several methods for mounting a rail to form a track for the railway engineer. For the following methods, state the advantages and disadvantages and describe where they would most likely be used:

(i) Timber sleeper

(ii) Steel sleeper

(iii) Mono-bloc concrete sleeper

(iv) Twin-bloc concrete sleeper

(v) Slab track

Answer 1

Railway sleepers

• They are the components on which the rails are arranged with a proper gauge. (such as broad gauge, narrow-gauge, etc..)
• These sleepers generally rest on the ballast. The load from rails when the train passes is taken by these sleepers and is distributed to the ballast.
• They are also known as ties in some regions.

Characteristics of Ideal Railway Sleepers

1. Initial cost and maintenance cost should be low
2. They should resist weathering, corrosion, decay and other deterioration
3. They should bear the wheel load efficiently and satisfactorily
4. They should maintain the correct gauge
5. They should absorb shocks or vibrations due to moving vehicles
6. It should distribute the load properly and uniformly over the ballast
7. Fastenings of rail with sleepers should be strong and simple
8. They should not break while packing of ballast
9. Weight should not be low or high

Classification of Railway Sleepers

Based on the materials used, railway sleepers are classified into the following types.

1. Wooden sleepers
2. Concrete sleepers
   • Mono-bloc concrete sleeper
   • Twin-bloc concrete sleeper
3. Steel sleepers
4. Cast iron sleepers
5. Composite sleepers

Wooden Sleepers

• These are sleepers made of wood.
• These are commonly 254mm wide by 127mm thick in cross-section by 2600 mm long.
• The sleepers are first seasoned (drying for up to 12 months so that to remove the juice/sap) and treated with preservatives. Creosote is an oil generally used/ sprayed on the surface.
• Wooden sleepers are used since olden days.
• These are still widely using in some western countries.
• Either hardwood or softwood can be used to make wooden sleepers. However, hardwood sleepers made of oak, jarrah, teakwood are more famous.

Two types of timber sleepers are

1. Switch Ties:

   The primary use for switch ties is to transfer load (as from the name) and are made of hardwood. This type is preferably used in bridge approaches, heavily traveled, railway crossovers, and transition ties.

2. Softwood Ties:

softwood timber is more rot (decay) resistant than hardwood, but does not offer resistance to spike hole enlargement, gauge spreading, also are not as effective in transmitting the load to the ballast section as the hardwood tie.

Softwood ties and hardwood ties should not be mixed on the main track. Softwood ties are typically used in open deck bridges.

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Advantages of Wooden Sleepers

1. Wood sleepers are cheaper than others and easy to manufacture.
2. They are light in weight so, they can be easily transported and handled while installing.
3. Fasteners can be easily installed to wooden sleepers.
   They are good insulators so, rails are well protected.
4. Any type of gauge can be maintained by wooden sleepers.
5. They are suitable for all types of rail sections.
6. They are well suitable for tracks in coastal areas.

Disadvantages of Wooden Sleepers

1. They are easily liable to attack by vermin and weather
2. They are susceptible to fire
3. It is difficult to maintain gauge in case of wooden sleepers
4. Scrap value is negligible
5. Their useful life is short about 12 to 15 years.

Steel Sleepers

• Steel sleepers are more often used because of stronger than wood and economical than concrete.
• They also have a good life span.
• They are molded in trough shape and placed on ballast in inversed trough shape.
• They are also used in heavy curvature prone to gauge widening.
• This type of steel ties can cause the problem to the signals control system.

In the design of Steel sleeper, the following are considered:

• It should maintain perfect gauge
• Can fix the rail and there should be no movement longitudinally
• Should have sufficient effective area to transfer load from rail to ballast.
• The metal of sleepers should be strong enough to resist bending
• The design life should be 35 years

Advantages of Steel Sleepers

• They are light in weight so, easy to transport, to place and to install.
• They are recyclable hence possess good scrap value.
• The life span of steel sleepers is more and is about 30 years.
• They are good resistant to fire.
• They have good resistance against creep
• They cannot be attacked by vermin etc.
• They are well suitable for tracks of high speeds and larger loads.
• They hold the rail strongly and connecting rail to the sleeper is also simple.

Dis-advantages of Steel Sleepers

• Steel can be affected by chemicals easily.
• Steel sleepers require high maintenance.
• They are not suitable for all types of ballast which are provided as a bed for sleepers.
• If derailment happens, they damaged very badly and not suitable for reuse.
• These are not suitable for all types of rail sections and gauges.

Concrete Sleepers

• Concrete sleepers are manufactured by concrete with internal reinforcement.
• Concrete sleepers used in many countries due to their high stability and small maintenance.
• These are more suitable for high-speed rails.
• Most of the concrete sleepers are made from pre-stressed concrete in which internal tension is induced into the sleeper before casting. Hence, the sleeper withstands well against high external pressure.
• They have a design life of up to 40 years.
• They can easily be molded into the required/design shape to withstand stresses induced by fast and heavy traffic.
• The added weight helps the rail to resist the forces produced due to thermal expansion and which can buckle the track. The weight of concrete sleepers is about 2.5 to 3 times the wooden sleepers.

Advantages of Concrete Sleepers

• Concrete sleepers are heavier than all other types hence, they give good stability to the rails.
• They have a long life span so, economically preferable.
• They have good Fire resistance.
• Corrosion does not occur in concrete sleepers.
• Vermin attack, decay, etc. do not occur. Hence, they are suitable for all types of soil and moisture conditions.
• Bucking strength is more.
• Concrete is a good insulator so, this type of sleeper is more suitable for circuited tracks.
• Concrete sleepers hold the track strongly and maintain gauge.

Disadvantages of Concrete Sleepers

• Because of heavyweight, handling is difficult.
• For tracks on bridges and at crossings, concrete sleepers are not suitable.
• Damage may occur while transporting.

Concrete sleepers are of two types:

• Mono-bloc concrete sleeper
• Twin-bloc concrete sleeper

Mono-bloc concrete sleeper

• These are the development of sleepers into prestressed sleepers that took place about the same time as the development of flat bottomed rail and direct fastenings.
• It is first in the UK prestressed concrete sleeper manufactured by the pretension method. Variations of this standard sleeper are available with extra holes for supporting conductor rail insulators and with shallower depth where these conditions apply. In this method, the prestressing tendons are tensioned prior to the concrete being placed and are only released once the concrete has reached sufficient compressive strength to resist the induced forces thus applied. This method is also sometimes referred to as the 'Long Line' system, as sleepers are cast five sleepers or more.
• Some counties outside the UK adopt the post-tensioning method where tendons are placed in debonding sheaths and the stress is applied after the concrete has hardened by the application of tensile force to the tendons by jacking and final anchoring. This method is slower but less capital intensive and lends itself to small-scale production and situations where demand is less.
• Standard prestressed concrete sleepers used in the UK are normally 2515mm long by 264mm wide. The depth varies from 203mm at the rail seat to 165mm at the centerline giving a total weight of 285 kg.
• The prestress is provided by six No. 9.3mm strands for standard use increased to eight and strands for heavy-duty.
• These sleepers are capable of sustaining an equivalent dynamic load of 24 tonnes at each rail seat. Allowing for impact, lurching, wheel flats, poor rail joints and etc this is equivalent to the effects of the passage of a static 25 tonnes axle.
• Metros and light rail systems have extensively also adopted prestressed concrete sleepers. Where maximum axle loads are less than for mainline, the sleeper dimensions may be reduced accordingly. However, great care must be taken in the design to ensure that adequate allowance is made for dynamic effects, particularly for bending moments.

Twin-bloc concrete sleeper

• The twin block sleeper consists of two reinforced concrete blocks joined together with a steel tie bar cast into the blocks.
• They are for ballastless slab track, related to a railway sleeper of the type to be used in particular for ballastless track or (concrete) slab track and comprising two concrete blocks joined by a reinforcing steel frame, said blocks being provided with holes intended to house a fastening to secure a rail, preferably four holes which allow the sleeper to be used for two different track gauges, thus being multipurpose, and in any case with the particularity that said sleeper blocks have an improved structural configuration which provides significant advantages, both in their manufacturing and in their effectiveness for absorbing displacements and loads of the fastenings which support the rails, with respect to the systems currently known for such purpose.
• This type of sleeper is used extensively in Europe, particularly in France, but not in the UK.
• The standard sleeper weighs 230 kg which is less than the monobloc equivalent.
• However, handling and placing can be difficult due to the tendency to twist when lifted.
• Twin block sleepers can be provided with resilient 'boots' and can be incorporated into non-ballasted slab track or monolithic embedment in road surfaces for light rail street running.

Slab track

• Slab track, also called the ballastless track, is a modern form of track construction which has been used successfully throughout the world for high-speed lines, heavy rail, light rail and tram systems.
• For this, first, we have to know what is a ballasted track -

On the ballasted track, the rail is mounted onto a wooden or concrete sleeper. The sleeper sits on a bed of ballast (crushed rock) which distributes the loading to the subgrade. “Top” ballast is placed between the sleepers and on the shoulders to provide longitudinal and lateral stability.

• Now, the slab track is a type of railway track infrastructure in which the traditional elastic combination of ties/sleepers and ballast is replaced by a rigid construction of concrete or asphalt.
• In ballastless tracks, the rails are rigidly fastened to special types of concrete ties/sleepers that are themselves set in concrete. Ballastless tracks, therefore, offer a high consistency in track geometry, adjusting of which is not possible after the concreting of the superstructure.

Concrete Slab Track

• With concrete track slab systems, the ballast is replaced by a rigid concrete track slab which transfers the load and provides track stability. Resilience is introduced into the track system by means of elastomeric components. These may be pads, bearings, or springs depending on the type of slab track system.

There are broadly five types of a generic slab track system:

• Embedded rail
• Booted sleepers
• Direct fixing and resilient baseplates
• Cast-in sleepers
• Floating slab

Within each generic group, there are a large number of variants and proprietary systems available. Slab track can be designed and optimized to suit the required application.

Advantages of slab track

Slab track offers the following advantages over the traditional ballasted track:

• Very low maintenance requirements
• Shallow construction depth
• Reduced dead load
• Reduced structure gauge
• Higher speed operation
• Engineered noise and vibration performance
• Long design life
• Increased reliability
• Increased availability
• Low whole-life cost
• A sustainable solution

Disadvantages of slab track

• Higher cost of initial construction.

While numbers vary depending on construction type and track infrastructure (ballastless tracks are generally more suitable to infrastructures that are also made of concrete, as is the case in tunnels or on viaducts)

It is estimated in 2015 that construction costs of ballastless tracks are 40 percent higher than those of traditional superstructure. However, the life-cycle cost of ballastless tracks is generally lower than those of ballasted tracks due to significantly lower maintenance.

• impossibility of adjusting or correcting track geometry once the concrete has been set
• the necessity of a stable infrastructure (since no adjustments can be made to the superstructure)
• higher noise emissions
• longer repair times when the concrete slab is damaged (e.g. due to construction faults or wear and tear).