Today's prompt for Sunday Stamps is 'urban'.
I don't think it gets much more urban than having a railway for mail that allowed for rapid movement under the city of London. This is a Royal Mail Post & Go postal label dispensed from a machine (in some countries they may be called stickers).
The post office railway was driverless and ran from 1927 to 2003. It is now a museum. It ran between Paddington and Whitechapel, was 6.5 miles (10.5 km) long and had eight stations. Each station was located underneath a sorting office. Over time, as sorting offices were relocated, only 3 stations remained in use. Finally, due to the fact that it was more expensive to run trains than to truck mail around London (although the trains were more environmentally friendly), the Mail Rail closed.
You can read more about this unique piece of postal history at its wikipedia page.
For links to more blogposts about stamps with an urban theme, head over to Sunday Stamps.