First conceived in the 1850s and built in it's current state in 1929, the railway was active for 74 years, before closing in 2003. History lesson over, onto the explore.
Broken down into two sections based on either side of the main HQ for the Railway, Mount Pleasant, the northern most middle pin in the centre of this map.

Explore wise the Post Office Railway had been on the list of explores in London for as long as exploring has been going. I can remember receiving a Ladybird book on trains as a kid (my dad being a fan of trains), and in it there were a few pages on something called Mail Rail. I didn't think I'd ever get to see it. Then in the tail end of 2010 it was invaded by some ravers holding a party in the former Royal Mail building above. This raised interest in a number of explorers, those from the LCC (LCC = exploring collective lead by Siologen and Brad Garrett), Lonewolf Speed, Kev & Gringz from my usual exploring trio. Kev had spent time at the Royal Mail archives as well walking the route. However other projects always got in the way. I had walked around the main HQ while searching for lids for the Fleet. However it was the LCC peeps that got in first, and they hit it numerous times over the course of a week, 2 months prior to my trip. Kev thought we had the right spot, a former building labelled as the Mail Rail building in Mt. Pleasant. However it turned out to be just an office with no connection to subterranean fun.

However it was to be East London where we got in. Kev had identified a shaft with possibilities. When it came to testing the defences of the live station, Kev was caught out with barely a leg over a wall by Security.

On a dark night I hoped it would be raining cats and dogs, as I had seen it on the 'always reliable' weather forecasts. As we got there it was spitting a little bit. Damn! People came and went, cars pulled out of the District office. People in dark navy jumpers walked by with Royal Mail badges on occasionally. I was getting jumpy, keen to go in, but also keen to avoid problems. Kev was in a similar position, but worse. He'd read bits on my site over time, but reading and doing are very different things. Eventually I felt I had a gap, and went for it. Leaping up a wall, hopping into a goods yard, Kev was quick on my heels, and we got closer to the goal. I checked we hadn't been spotted, and anxious not to hang around, dropped into the next property, the E@stern D!strict Office. Due to the beauty of plans available at archives, and Bing maps, I landed a few metres from the target. A vent with mesh cover. Luckily it was relatively easy to squeeze through, and drag my battered camera bag through after. Kev had been peeking over the wall from the goods yard, and when I gave a final OK, he joined me. This wasn't the hard bit, the hard bit was descending down a 20m shaft to the POR station below.

As Kev climbed in, I was stood on the vent frame with a bemused look on my face. Pointing a torch down the hole, "whaddya think?!" "Erm" being the reply back. I had no idea what to expect, as I entered the vent. I suggested I would go down a metre or so, and if it seemed ok, would keep going to the bottom. This is where I really could have done with knowing rope skills. I carefully descended a metre or two, and the large bolts proved easy enough to hold onto. Knowing a slip would mean death or severe disablement, I took my time with Kev lighting the descent. It took a good quarter of an hour or so, but reaching the bottom I was dripping with sweat. Not from the climb but the sheer terror which was only now kicking in. From the now weak torch light, I could only see a bricked up wall with a door in it, I wasn't hopeful. I broke out my backup torch and tried the door. Thankfully it opened into a dank area, with steps up. I nervously poked my head up the steps to find I'd found what I wanted. I went back to Kev who'd already begun his descent using a head torch to guide his movements. I was even more nervous about his descent than mine. But equally I didn't stand underneath him! Thankfully he made it with a wild look in his eyes. "Fuck!" he shouted. "Yes" I replied back with a high five, before remembering where we were and quickly exiting the vent.

Whitechapel - Eastern District Office

I put on my snood to hide my face from the many cctv cameras I'd been told to expect, and Kev went for a stylish black silk balaclava, which scared me somewhat. I didn't want to be caught in the tunnels with a rapist. Now, one of the things I'd seen on various Post Office Railway sites, is a VIP train, that intrigued me. I hadn't seen it on any of the LCC's trips, but felt it must be there somewhere. The first thing I saw as the infrared lights came on. in the station was the VIP train. This was all very weird. Kev made one of the conditions of coming that his face couldn't be recognised on my tawdry website, and that I wouldn't take the piss out of this. Well, 1 out of 2 isn't bad.

A complex high technology board sat over the tracks showing train movement.

There was also a board (which was repeated at most of the more recent stations to close) above the tracks, reminding staff of the golden rules.

Looking to find the most Easterly point, I disappeared into the tunnels for the first time and came to a junction...

The Loop was quite literally that, a loop back around to the start of the other platform. The other passage was what I was after.

It was a siding with my first glimpse at a once operational train. The driverless mail rail trains reached 30mph in the tunnels.

Just past the train above was the end of the siding tunnel, and the most Easterly point. From here it was a long walk, and a long night ahead.

At the other end of the platform I passed Kev trying to figure out how his camera worked.  Just behind Kev was a set of stairs that lead up to the surface in another vent. This had a spiral mailbag chute.

Bottom of the chute, which goes out to the platform.

It was then the start of the great march west, 6.5miles of walking. I hadn't prepared for this at all, and we only had about 250ml of water between us. No stay awake pills, no mountain dew. It was around 2.30am now. Oh well. Nothing ventured and all that.
The walk wasn't that bad as long as the strip lights were to the side, when they were overhead I had to duck, which was a pain. Some shameless site pimping i'm afraid!

Liverpool St Station

This was our next stop, the rumble through the earth showed we were at the right place, as mainline trains rolled by overhead. I brushed the countless bits of stalactites from my hair, and stood at the entrance to the station...below the cctv camera. I don't know why, as it was impossible to avoid, but we ran into the station to the first crossover. As soon as we crossed the threshold, a light came on that illuminated half the station for the cctv.
Similar to the previous station, with panels covering the arches of the station tunnel. A sign on the wall reminding dopey workers not to chuck tea bags on the tracks.

The control room at Liverpool St. Not particularly high tech. A bit like air traffic control, one station controller would pass the train onto the next station. By the end of the Mail Rail's life, they were all controlled from a single control point by computer.

Various alarms, including the rather worrying 'Smoke under East/West Platform' lights. Wouldn't want to be around for that.

Also like the preceding station, there were conveyor belts to the surface, although due to the fact it was a mainline station, there were two belts down, not one. The belts also went on for some way, probably due to the fact the Post Office Railway station would have to be sited away from all the other things below Liverpool St. Station, including tube activity.

A ventilation pipe goes to the surface above the conveyor belts, at the top one could hear people near food outlets ordering food?!

The second leg of conveyor belts from the station, about 50m long.

Here the belts became a double helix, and went up a shaft to the surface. A previous explorer had complained of thigh deep water at this point, I'd never realised how short he was before! It was only about 10 centimetres deep at best. I scuttled along the edge of the belt apparatus, and hopped up the steps.

The double helix conveyor belts going up.

Not expecting much more than a door and a pir, I left my camera down at the bottom. But there was more. This is the chute from another conveyor belt that runs right of this photo, and drops bags into the double helix below. Pics taken on mobile phone.

Turning right the belts were bright blue and went on for about 20-30m before coming to a fenced off section. Through the fence I could see about 10m of more conveyor belt, and then chutes above them where bags would have been dropped in. The belts were dismantled to put the mesh fence in.

Back on the station, the green conveyor belt would have carried bags from the belts mentioned above. Dropped into the mini-york carts, and popped onto trains.

So far we'd done two stations, and I was feeling proud we'd got so far. If we got busted now, I wouldn't care. Although the completest in me would want to return. Back into the tunnels, and more ducking and weaving to the next station.

King Edward Building

Long out of use, the station had had it's platforms boarded in.

Nothing much of interest behind the boardings. The curved wood on the right covers a former exit to the surface. The building above is owned by an investment bank now. Kev wondered if he could gain intel, layers of concrete said no.

I did find a newspaper from 1999, talking about the Dome being completed on time.

At each end of the platform, stairs lead down under the tracks. I didn't hang around long here, as I could smell bad air and saw lots of fungus about, that i'd seen in quarantined areas at Burl!ngton city.

Nothing particularly exciting, Special Notices or otherwise.

Once again, it was time to push on, a jaunty three quarter of a mile walk to the next station. This would be the real test of how things were down here, as the next station was very much still live above the surface, and also the HQ of the Mail Rail. It would also be the midway point of our trip.

Mount Pleasant Station

After bumbling along the tunnels for awhile, we came to a number of trains laid up here. It was the largest number I saw in anyone one concentration.

Lots of the trains all stored in various states in the tunnel.

We came through the heavy door on the left, of pretty much bomb proof design. On the right was the Loop tunnel, which would allow trains to go around to the other platform. Strangely, on the wall of the Loop were drawn children's story type paintings.

The Children's drawings were done for children's parties apparently, held with the profits made from letting Bruce Willis make a dreadful film down here.

The Westbound track from the top end...

And looking back up the Westbound platform. We could hear people clanging things about above. At one point we were convinced a lift had ended up at the platform level and bolted into the tunnel to see what was happening. But it must have been the floor above.

Chute exits on the Eastbound platform.

A map of all the station layouts on the Mail Rail. Two of the Western long derelict stations are missing after Paddington.

We looked around for some way of getting to the repair workshops I was keen to see. However there didn't seem to be anything obvious. Kev wandered up a track and came back to say he'd found it. We both returned to the upwards curving track that led up to the workshop area. Not knowing how far we were away from being discovered, we almost quite literally tiptoed around. Doing our best to dodge the various cameras and a pir on a wall.

One can imagine getting a job here for a train engineer would be seventh heaven. A crane sits in the middle of this room, ready to do the heavy work.

Countless spanners and hex bolts in the trays.

Not wishing to hang around any longer than necessary, it was time to hit the Eastern section. Again another huge door marked our departure point, and another 15 minute walk to the next station.

All the stalactites ready to drop onto your hair. When I got back and washed my hair, the bath was sprayed with brown water. Three tunnels of descending height, the largest being the one we're in, the medium tunnel led West, and if memory serves, the smaller one lead to the repair shop or was the loop track.

And with that, the end of the Eastern section. The Western section being more of the same, but with some stations more derelict than the Eastern ones. Read on here.

There are some good history sites for those interested in further reading.
One run by some train enthusiasts, here

Some bits on sub-brit
A first hand account on the BBC of a controller who worked on the Mail Rail.
There are also some great videos, showing and explaining the system working.  here and Here.
This is one I knocked up

Mail Rail from guerrillaexploring on Vimeo.