Editing William Hedley (section)

==Hedley's improvements==

===First locomotive===
Hedley felt that if the pairs of wheels were connected, as with [[Richard Trevithick]]'s engines, if one pair began to slip, it would be counteracted by the other. The mine owner, [[Christopher Blackett]] had just replaced the wooden [[waggonway]] with iron flanged 'L' section plate rails. Hedley first constructed a test carriage operated by manpower, to test the [[Rail adhesion|adhesion]] under various loads. He then used it as the chassis for a locomotive constructed to Trevithick's pattern with a single cylinder and a simple straight through fire tube to the boiler.  This engine was not satisfactory. Its motion was erratic, because of the single cylinder, and it produced insufficient steam.

===''Puffing Billy'' and ''Wylam Dilly''===
He built a second engine, with the assistance of the, later to be famous, [[Timothy Hackworth]], his foreman smith, and his principal engine wright, Jonathan Forster, using the 1812 twin cylinder plan of [[John Blenkinsop]] and [[Matthew Murray]] and a [[return flue boiler]]. This was the famous [[steam locomotive]], ''[[Puffing Billy (locomotive)|Puffing Billy]]'' which first ran in 1813 and is now preserved at the [[Science Museum (London)|Science Museum]] in [[London]]. Its success encouraged them to build a second engine ''[[Wylam Dilly]]'', which is now in the [[National Museum of Scotland]] in [[Edinburgh]]. In the same year, his system for using a coupling between the wheels was patented.

===Modifications===
However, there was still considerable wear to the track, and the engines were rebuilt using twin four-wheeled bogies, introduced in Blackwell's design mentioned above. Initially the wheels were without flanges for use on the flanged plate rails. In about 1830 the line was relaid with the stronger edge rails, and both locomotives reverted to their original pattern, but with flanged wheels, which is how they are today. Both locos remained in active service until 1862.