cardiff. for 75 years, an underground network of railways was used by royal mail to move post around the capital of the public will be able to take a unique ride through tunnels previously travelled by only parcels and letters. tim muffettjumped on board. throughout its 500—year history, the royal mail's mission has remained pretty much unchanged. newsreel: now down the chute into the vans... to harness technology of the day to deliver letters and parcels as quickly and accurately as possible. this new postal museum shows how deliveries have evolved. but by the early 20th century, the mail system in london faced two big problems — heavy fog caused by smoke billowing from chimneys, and heavy traffic. the answer lay below ground. in 1927, this underground rail network opened. newsreel: on the post office tube railway, 25,000 mail bags a day travel through 6.5 miles of tunnels below crowded city pavements... for 75 years, unmanned trains shuttled mail between six sorting offices and two railway stations, liverpool street and paddington. i guess it was designed for letters, not people. exactly right