A sleuthfully explored documentary of London's mail handling underground railway.
http://www.silentuk.com/?p=2792

A fascinating look at a system that's apparently out of service. There's a website devoted to "urban explorers" who visit abandoned railway facilities and other places that time has passed by. One photo study even showed a visit to a closed-down mental hospital. What was surprising in the UK Mail Rail exploration was how well illuminated many of the photos were; it did mention that the explorers found a lighting control panel at one point, but the fact that the lights were still working was interesting. Some of the underground areas looked like the workers had just stepped out for tea-time. For those of us who are not as adventurous, the National Railway Museum in York has an exhibit devoted to the postal subway system (at least they did in 1993). Note that the lines shown in the photo study have a third rail--looking like "The Jolly Green Giant's Lionel trains."

It's not a big "mystery". The mail rail system was in Trains when it closed ten years ago. It was closed, not scrapped. Why wouldn't all the lights still work?