Electrifying railways from the ground up - Rail101

Powering railways

Electricity is generally accepted to be the the most reliable and most efficient way to power trains of all types.

However, as trains don't generally run with a giant extension lead trailing behind them, the electrical power is transferred to the train via either overhead wires, or via rails carrying electricity at ground level (or very occasionally overhead rails). There are occasional uses of battery and super capacitors too but lets look at the more mainstream options.

In this brief #rail101 article I take a look at conductor rails, and why they are different to most railways rails.

Conductor rails carry electricity to power the train and are sometimes called third (3rd) or even fourth (4th) rails, as the extra electric feeding rails are in addition to the normal "running" rails that the train rolls along. Conductor rails must carry large currents necessary to power trains on railways and metro systems. Usually operating at 1500v DC or below, conductor rails are particularly suited to tunnel operations where the lower space requirements compared to overhead wires allow smaller tunnels to be bored, (which are cheaper and faster to build). For this reason they are usually (but not exclusively) found in underground and metro applications.

To transfer the electrical current between the rail and the train a "shoe" mounted on the train slides over the surface of the rail as it travels. There are various designs where the shoe slides over the top, bottom or side of the rail - each have their merits and challenges. If you see a bright electrical flash as a metro train passes by this is usually due to the shoe making/breaking contact briefly with the conductor rail. The picture above shows how flat the top of the rail is - specifically designed to ensure a good contact area between the shoe and the rail.

Traditional railway "running" rails support the vehicle weight and transmit the forces needed to steer, accelerate and slow trains down. Because of this requirement for high load bearing and wear resistance, "running" rails are pretty hard and strong.

However if we look at conductor rails (which are additional to the running rails), their primary requirement is high electrical current carrying capacity to power the train. The usual alloy additions made to rail steels for improved hardness and wear resistance also increase the electrical resistance of the steel - not what we want for this application. To carry electrical currents efficiently a very different type of steel is used.

The steel composition of conductor rails is designed to provide the minimum electrical resistance possible (or maximum conductivity if you prefer) to allow efficient power supply. For this reason conductor rails are almost pure iron. Where a running rail may have 0.7% carbon in it a conductor rail is usually below 0.007% - One hundred times lower!

The composition change means the electrical resistivity of a steel conductor rail is typically half that of a standard "running" rail, and its mechanical properties are very different, with the hardness of the steel being much lower than that of a traditional railway rail.

In the quest for lower resistance/smaller space profile still, aluminium rails are also used in some proprietary conductor rail systems with a stainless steel cap/wear strip on the surface to resist the sliding contact of the trains over the rail in service. Here is an example from REHAU

Often conductor rails are used on metro systems which tend to have a higher proportion of tunnels and sharp curves, so shorter length rails may be accepted and may even make installation easier in some cases. However when used in more open routes at higher speeds (N.B. not high speed) an increased length of rail can be beneficial to minimise the discontinuities as joints in the rail are avoided and speed rail replacement.

Below is a little video I shot of 600ft long conductor rails - each one weighs nearly 14t! I don't think you'll find any that are longer!

Round up?

• Conductor rails need to perform a very different function to traditional "running" rails and so are specifically designed to minimise their electrical resistance
• The electrical resistance (resistivity) of steel conductor rails is typically half that of a traditional rail. Aluminium rails can achieve even lower resistance.
• Conductor rails are REALLY soft compared to railway rails being made of almost pure iron or in some cases aluminium.

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