Railway overhead electrification system design

The railway overhead electrification systems are the key part of Modern railroad infrastructure which gives electric trains a reliable and efficient power supply. The prominent feature of the Overhead electrification system is the overhead catenary system, which consists of a wire strung above the tracks to adequately provide electrical power to the trains.

The figure below shows the classification of electrification power that is currently under use in different areas of the world.

OLE voltage of different geographical regions

The goal of railway overhead electrification systems is to give electric trains a durable, sustainable, dependable, and secure power source that will enable them to run at high speeds.

Trams, trolleybuses, and trains receive electrical electricity from overhead lines or wires that are located far from the source of the energy. There is a variation of nomenclature used to name those structures.

Various names for these overhead lines include:

Overhead contact system (OCS)—Europe, excluding the United Kingdom and Spain

OLE or OHLE: overhead line equipment (UK)

OHE: United Kingdom, India, Pakistan, and Malaysia

Catenary: United States, India, United Kingdom, Singapore (North East MRT Line), Canada, and Spain. Overhead wiring (OHW)—Australia.

Rail Overhead Electrification System Components

 A rail overhead electrification system is made up of a number of parts. The length of OLE systems sections ranges from 1320m (Sweden) and 1500m (UK). The sections have overlap sections of 180m where the contact wire changes sides.

A number of wires are stretched above the tracks and are referred to as overhead lines or overhead catenary. The primary element of the railway overhead electrification system, the overhead catenary system consists of a wire stretched above the tracks. The wire is often supported by a number of masts or poles and is made of steel or aluminum.

The overhead catenary wire consists of a messenger or catenary and contact wire. It is essential to maintain the contact wire geometry within predetermined bounds in order to accomplish good high-speed current collection. Typically, to do this, a second wire known as the messenger wire (US & Canada) or catenary is used to support the contact wire from above (UK). 

This wire is sometimes referred to as a catenary because it resembles the natural course of a wire strung between two points or a catenary curve. 

The messenger wire is used to support the overhead catenary wire of an overhead railway. The overhead catenary wire is the wire that carries the electrical current to the trains, and it is suspended above the tracks by a series of poles. It serves as a support for the catenary wire and helps to keep it in place. It also helps to distribute the weight of the catenary wire more evenly along the length of the track, which can help to reduce the strain on the poles and the wire itself.

Vertical wires called droppers or drop wires: are used to connect this wire to the contact wire at predetermined intervals. A pulley, link, or clamp is frequently used in constructions to support the messenger wire. The system as a whole is then put under mechanical tension.

Contact Wire: This is the wire that carries electrical power to the pantograph and is suspended above the tracks. The contact wire is often supported by a network of masts or poles and is composed of steel or aluminum.

Cantilevers: The protruding section that hangs the contact wire from the posts is called a cantilever. They support the registration arm that hangs the contact wire. Cantilevers are single in normal sections but become double in common sections because the contact wire changes sides and is lifted to be fixed on the posts on the sides. Cantilevers consist of pull-off or push-off arms depending on their location. The pull-off arms pull the contact wire towards the mast. Push-off arm pushes the contact wire away from the mast.

The registration arm is used in an overhead railway catenary system to ensure proper alignment and spacing of the catenary wires, which provide power to the trains. It is a mechanical device that is mounted on the catenary wire and can be moved along the wire to adjust its position. The registration arm helps to maintain the proper tension and spacing of the catenary wires, which is essential for the smooth operation of the trains and the safety of passengers. It is also used to maintain the proper height of the catenary wires above the tracks, ensuring that they do not come into contact with the trains or other objects on the tracks.

The registration arm makes the contact wires elastic so that the movements of the contact wires do not create additional tension. This provides elasticity to the catenary system and it dampens any changes of levels that occur during the course of the train.

A pantograph is a device that collects power from the overhead catenary system and is positioned on the roof of an electric train. The pantograph, which is made of a flexible metal strip linked to a spring-loaded arm, can rise and fall with the movement of the train as it travels along the rails.

The carbon surface of the insert atop the pantograph wears away as the contact wire makes contact with it. As the train passes around a curve, the “straight” wire between the supports will force the contact wire to cross the whole pantograph surface, causing even wear and preventing any notches. In order to ensure that the pantograph wears uniformly on a straight track, the contact wire is gently zigzagged to the left and right of the center at each succeeding support.

Through a pantograph, a mechanism located on the train’s roof, these lines deliver electricity to the trains. Usually composed of steel or aluminum, the overhead cables are held up by poles or masts.

The posts or masts in an overhead railway catenary system are used to support the wires or cables that carry the electrical power to the trains. The posts are typically made of steel or concrete and are spaced at intervals along the track. The wire or cables are attached to the posts via insulators, which help to prevent electrical grounding and ensure a stable power supply to the trains. The posts or masts also help to maintain the correct height and tension of the wire or cables, which is essential for the safe and efficient operation of the trains. 

Tensioning system: The pantograph creates oscillations in the catenary wires, and in order to prevent standing waves from forming that could destroy the wires, the wave must go faster than the train.

As the line is tightened, the waves move more quickly. The wires are typically tensioned by weights or sporadically by hydraulic tensioners for medium and high speeds.

Both techniques, collectively referred to as auto-tensioning (AT) or constant tension, guarantee that the tension in the apparatus is essentially temperature-independent.

Typically, tensions per wire range from 9 to 20 kN in the UK; 4.5 to 15 KN in Sweden.. To prevent the weights from wobbling while they are utilized, they glide up and down on a rod or tube linked to the mast. 

Fixed termination (FT) equipment, where the wires are terminated directly on buildings at both ends of the overhead line, may be employed for low speeds and in tunnels with consistent temperatures.

There is a maximum continuous length of the overhead line that can be installed when AT is employed. This is because as the temperature changes and the overhead line expands and contracts, the position of the weights changes. The tension length, or the distance between anchors, determines how much movement there is. The idea of maximum tension length follows from this. The maximum tension length in the UK is 1970m, and in Sweden is 1370m  for the majority of OHL equipment.

A further problem with AT equipment is that the entire tension length will be free to travel around the track if balance weights are not attached to both ends. To solve this problem, a midpoint anchor (MPA) in the tension length’s center anchors the messenger wire while limiting its movement; the contact wire and its suspension hangers can only move within the limits of the MPA. 

In other cases, MPAs are fastened to the standard vertical catenary poles or portal catenary supports. MPAs are occasionally fixed to low bridges. 

As a result, a tension length may be thought of as having a fixed center and two half-tension lengths that expand and contract in response to temperature.

Tensioning wires are about 1500m (UK) or 1320m(Sweden) but can vary according to circumstances. The tension wires ends are tensioned at the ends. There is an overlap section that is used to sustain a continuous power supply and where the wire changes sides. The length of a section is considered as the distance between the overlapping sections. 

Railway Overhead Electrification System Design Considerations

When designing a railway overhead electrification system, there are a number of important factors that are taken into account. The objective of the design is to create a system that can enable a continuous supply of electricity and withstand wind conditions and fluctuations in temperature.

Electrical requirements: The electrical needs of the electric trains that will be utilizing the system come first. The kind, size, and operating conditions of the trains, as well as the required electrical power for the trains, must all be delivered by the overhead catenary system. Mostly Overhead catenary systems are suited for AC systems and third rail systems are supplied with DC current. That is because DC currents are safer and more controllable.

Track Geometry: The overhead catenary system must be created to accommodate the curve and gradient of the tracks as well as the track geometry. This will guarantee that the system can supply the trains with a steady source of electricity and that the pantograph can always retain contact with the contact wire.

Determination of Alignment of contact wire: The contact wire is provided in the zigzag form called stagger, for straight line or horizontal alignment with a large radius.

However, when the radius of horizontal alignment is small for example less than 5000m, the alignment is provided on the outer side of the radius. 

Courtesy: Swedish railway standard

The reason is to ensure that contact with the pantograph remains sufficient during the course of the train. The distance of the contact wire from the horizontal alignment depends on the maximum windspeed allowed and the tension in the cable. The maximum values of wind speed and tension must not be exceeded.

The alignment of the contact wire becomes more complex in overlap sections because the contact wire changes sides and the part lifts up to be fixed on the posts on the corresponding side. In the same way, the contact wire has a more complex arrangement in turnouts since the contact wire’s alignment changes from single to double.  

Selection of cantilevers: The type of cantilevers in overlap sections is different than in the rest of the OLE section. That is because in the overlap section, there are two types of wires that need to be carried separately. The contact wire changes sides and it becomes fixed to the corresponding post.  Hence, double cantilevers are used to carry the two wires.

Clearance Requirements: When planning the overhead catenary system, it is important to keep in mind that there are usually minimum clearance requirements. These specifications guarantee that the system is safe for maintenance personnel to access and that it does not interfere with other infrastructures, such as bridges and tunnels. A suitable standard of guage profiles is used to determine the clearance requirements.

Horizontal line height: To prevent interference with adjacent structures or impediments, the height of the overhead lines needs to be carefully examined. In order for the pantograph to make contact with the overhead lines, they must be high enough, but not too high that they are challenging to maintain.

Distance between posts/masts: The distance between the posts/masts is determined to be the distance that is optimal to ensure the catenary is held straight in between and that which is economical. In Sweden, the maximum distance is 60m. However, the distance can vary and it is dependent on the stress in the cable, wind speed, and the radius of the horizontal alignment. The Distance between posts/masts and the distance of alignment of the contact wire is selected so that maximum values of wind speed and stress must not be exceeded.

Masts or portal frames: The supporting structure of OLE systems can be masts for single-track lines. Multiple lines may require portal frames in order to prevent congestion of the area and avoid placing masts in undesired positions. The main factor that determines the design is the wind speed that affects the system.

Foundations of masts or portal frames: The foundations of the masts or portal frames are selected based on the condition of the soil and the availability of material. Normally, prefabricated concrete sections are used in stiff soil conditions. If the soil is soft, bored foundations may be a suitable choice.

Example of standard values from Sweden’s systems

Maximum distance between posts: 60m

Maximum distance of contact wire from CL: 400mm

Maximum distance between tensioning points: 1320m

Maximum distance of anchoring point: 750m 

Tension in the registration arm is between 70-900 N

Railway technical word definitions

Alternating Current (AC) 

Electrical current that changes direction 50 times per second. 

Autotransformer Feeder System (ATF) 

System to be used for supplying power to the OLE. Incorporates ATF cables, generally one per track, attached to OLE masts and connected to autotransformer stations at intervals alongside the track. 


OLE structure comprising horizontal or near horizontal members supporting the catenary projecting from a single mast on one side of the track (see diagram on opposite page). 


 The longitudinal wire supports the contact wire. 


Any insulated wire, cable, or bar that carries electric current. 

Contact wire 

Carries the electricity which is supplied to the train by its pantograph.

Contact & catenary wire tensioning 

In order to keep the wires taut, they are in lengths of no more than c.1500m and tensioned at each end. 

Direct Current (DC) 

Electrical current that flows in one direction, like that from a battery. 

Dropper Wires 

suspended vertically from the catenary at regular intervals to support the contact wire.

 Feeder station

A facility next to National Grid electricity transmission lines extracts 25,000V and transmits it to the railway. The spacing of these stations depends on the electrification system used.


Components that separate electrically live parts of the OLE from other structural elements and the earth. Traditionally ceramic, today they are often synthetic materials.

 Kinematic envelope 

The space that defines the train and all its allowable movements – rocking, swaying, bouncing, for example. 

Loading gauge

 (vehicle gauge) The dimensions – height, and width – to which trains must conform in order to avoid colliding with line-side structures such as bridges and platforms. 


Trackside column, normally steel, that supports the OLE. Midpoint anchor At the midpoint of the standard length of OLE wires, the wires are fixed in position to keep the contact wire stable

Neutral section 

A length of electrically isolated or non-conducting material is incorporated into the contact wire to completely separate electrical sections of OLE. It may take the form of a short insertion in the contact wire or of an extended overlap. 


Overhead line electrification equipment, which supplies electric power to the trains. 


Each length of the contact wire overlaps with the next so that the pantograph slides smoothly from one to the other. 


The device on top of the train collects electric current from the contact wire to power the train. 

Structure gauge 

The defined space into which a structure must not intrude, to avoid trains colliding with it. This is larger than the kinematic envelope and loading gauge. 

Third rail system 

Railway electrification system using a third rail located alongside the track to supply DC power to the trains. No longer permitted new installations on national railways