From Margaret - A bit of history - with a walk - if you feel like it.

Causey Arch and the Tanfield Railway
Starting at the Causey Arch Picnic Area, this delightful walk follows the line of the Tanfield Railway, along the picturesque valley of the Causey Burn, before crossing the oldest surviving railway bridge in the world along the course of an old waggonway which was used to transport coal to the river.
The Railway was opened in 1725, and is now the oldest operating railway in the world.  It was constructed to take coal from the pits around Stanley and Marley Hill to the River Tyne.  Originally, waggons, or more correctly, chaldrons, filled with coal, were hauled along a wooden track by horses before being replaced by metal rails and static winding engines in the early 1830s, and by locomotives in 1881.  (A chaldron was an English measure of dry volume, mostly used for coal; the word itself is an obsolete spelling of cauldron.)  Most of the line was closed in 1970 but it is now run by a group of steam enthusiasts.
Causey Arch is the oldest surviving single arch railway bridge in the world and spans the gorge of Causey Burn.  Constructed in 1725-6 to provide a link between collieries at Tanfield and the main waggonway to the River Tyne.  The original design of a wooden track was crude, but nevertheless, effective.
It was commissioned by a powerful group of local coal owners known as the “Grand Allies.”  The Arch was designed by Ralph Wood, a local Stonemason, has a span of 100 feet and stands 80 feet above the valley floor.  Tradition has it that Wood was very apprehensive about an earlier timber bridge which had collapsed!!  Fearing that a similar fate awaited the stone structure, he leapt to his death from the top of the Arch.
On the western side there was a Toll House, where lines to other pits branched off, the remains of which are still in evidence.
There is ample parking at the Causey Arch Picnic Area; or arrival by public transport can be achieved via Stanley.  Great care is required at all times:  the train tracks are to be crossed three times; there are tree roots, steep sections and tall steps to be negotiated, but on a lovely sunny day, it is a magical walk – walking back in time itself.
Enjoy