Railway Electrification Voltages 

This post is quick introduction and overview to different railway electrification voltages used in answer to a question sent in via email.

While there are numerous voltages in use through out the world (many dating back a hundred years), fortunately the number of voltages levels in common/widespread use is relatively few. To keep the post short, I will only list the more common voltages.

Both dc and ac systems are used. Direct current systems are generally used for metro lines and relatively commuter /regional systems up to around 100 km long. For longer and more heavily loaded routes, alternating current is generally used.

Direct Current (dc) Voltages

Most commonly used are 600/650 and 750 V. These are used world wide on mass transit, commuter and tram lines. Voltage is supplied to the train using a third rail or overhead lines.

1.5 kV (typically supplied overhead) is used in Europe, the US, China, Australia and a several other countries. Less common are 3 kV systems, but these can still be found in operation in several countries.

Alternating Current (ac) Voltages

25 kV (50 Hz or 60 Hz) ac systems have achieved widespread use in many countries, including the UK, US, large parts of Europe, Asia, Africa and the Americas. Additionally 15 kV (16 2/3 Hz) networks still have widespread use within Europe and the US still has several 12 kV (25 Hz) lines.

The US, South Africa and Canada also operate 50 kV systems in operation (typically used on systems for mine haulage).

It would be fair to say that 25 kV has become the international standard. This is covered by two standards:

• BS EN 50163 - Railway applications. Supply voltages of traction systems
• IEC 60850 - Railway Applications. Supply voltages of traction systems

Alternating current systems invariably are supplied by overhead lines. It is worth noting that most implementations are single phase (due to problems with picking up three phases from the overhead lines).  

That's it; a quick summary. It doesn't cover every voltage, but hopefully will give everyone a general understanding of the most common voltages used.

If anyone has anything to add or want to give a few more details, please do so below.