Archivist Matt Tantony investigates the tech that kept Mail Rail moving.

I was exploring our Archive the other day, when I found some old photographs of this amazing bit of technology. It used to lurk in the Mail Rail tunnels, although it’s long gone now. But what on Earth is it? And what did it do?

A mercury arc rectifier, Mail Rail, 1973

A mercury arc rectifier in Mail Rail, 1973. POST 118/CT00358.

It’s called a mercury arc rectifier. It’s a large glass bulb, and at the bottom of it is a puddle of liquid mercury. And it really does glow bright blue when switched on. There were several mercury arc rectifiers in Mail Rail. They played a vital role in powering the Railway, from 1959 until the mid-1980s.

A mercury arc rectifier, 1973. POST 118/CT00358.
Post Office (London) Railway control desk, Mount Pleasant, 1973. POST 118/CT00358.
Loading a Post Office (London) Railway train, 1973. POST 118/CT00358.
The Post Office (London) Railway car depot at Mount Pleasant (the site of our Mail Rail galleries), 1973. POST 118/CT00358.
Unloading a Post Office (London) Railway train, 1973. POST 118/CT00358.
Post Office (London) Railway train indicator board, 1973. POST 118/CT00358.

Mail Rail was an electric railway. The trains were powered by direct current (DC) in the rails – 150 Volts in the stations, and 440 Volts in the tunnels. The level of electric current in the rails controlled the train speed. The electricity that was supplied to the Railway, however, was alternating current (AC), at a much higher voltage, drawn from the National Grid. Mail Rail had its own power sub-stations which converted the current to the required voltage. It was in these sub-stations that the mercury arc rectifiers did their very clever job.

Diagram of a mercury arc rectifier (source:

Diagram of a mercury arc rectifier (source:

Here’s the science part:

  1. There’s a pool of liquid mercury at the bottom of the glass bulb. Mercury is a metal that’s liquid at room temperature, and it conducts electricity.
  2. The mercury is heated. It partially vaporises and fills the bulb in gas form.
  3. The alternating current is fed in through the anodes (carbon rods) on the ‘arms’ of the bulb.
  4. The electric current from the carbon anodes ionises the mercury gas and arcs into the liquid mercury pool (the cathode).
  5. Since carbon and mercury behave differently when electrified, the current can only flow in one direction through the bulb: from the carbon anode to the mercury cathode.
  6. VoilĂ ! Electricity enters the rectifier as alternating current, and leaves as direct current.


There aren’t many of these gorgeous machines left in the world, but you can see two of them at nearby Kempton Steam Museum. Below is a video of a working model in the USA.

Mail Rail was planned from its origin to be a remote-controlled electric railway. We have lots of technical diagrams and photographs from its early days in our Archive. Below is a 1928 diagram of the Railway’s power supply system. Can you see the original eight stations in the top row? They run from Paddington on the left to the Eastern District Office on the right. The original electric power converter plants are at the bottom of the diagram.

Post Office Tube Railway, London: Supply and Control Arrangements, 1928 (POST 20/20).

Post Office Tube Railway, London: Supply and Control Arrangements, 1928 (POST 20/20).

And here’s one of the original rotary converters, photographed way back in 1926. This did the same job as the later mercury arc rectifier, converting AC to DC, but it did the job mechanically using a spinning dynamo. Click here to see over 150 more Mail Rail photographs from our Archive.

Rotary converter in Mount Pleasant Sub-Station, 1926. POST 118/5072.

Rotary converter in Mount Pleasant Sub-Station, 1926. POST 118/5072.

And finally… what powers our Mail Rail attraction trains today? Whereas the original Railway was powered through an electrified rail, our trains are 100% battery powered. Each train has a 72 Volt battery pack, consisting of 24 off 6 Volt absorbent glass mat cells connected in a series/parallel configuration. We charge them up overnight, ready to take another day’s visitors around the Mail Rail tunnels.

A photograph of a green miniature Mail Rail train travelling through an underground tunnel.

Mail Rail at The Postal Museum

If you haven’t visited yet, you can find more information here.

– Matt Tantony, Archivist (Cataloguing)