or charcoal needed to produce pig iron. Wrought iron was a soft material that contained slag or dross. The softness and dross tended to make iron rails distort and delaminate and they lasted less than 10 years. Sometimes they lasted as little as one year under high traffic. All these developments in the production of iron eventually led to replacement of composite wood/iron rails with superior all-iron rails. The introduction of the Bessemer process, enabling steel to be made inexpensively, led to the era of great expansion of railways that began in the late 1860s. Steel rails lasted several times longer than iron. Steel rails made heavier locomotives possible, allowing for longer trains and improving the productivity of railroads. The Bessemer process introduced nitrogen into the steel, which caused the steel to become brittle with age. The open hearth furnace began to replace the Bessemer process near the end of the 19th century, improving the quality of steel and further reducing costs. Thus steel completely replaced the use of iron in rails, becoming standard for all railways. The first passenger horsecar or tram, Swansea and Mumbles Railway was opened between Swansea and Mumbles in Wales in 1807. Horses remained the preferable mode for tram transport even after the arrival of steam engines until the end of the 19th century, because they were cleaner compared to steam-driven trams which caused smoke in city streets. Steam power introduced In 1784 James Watt, a Scottish inventor and mechanical engineer, patented a design for a steam locomotive. Watt had improved the steam engine of Thomas Newcomen, hitherto used to pump water out of mines, and developed a reciprocating engine in 1769 capable of powering a wheel. This was a large stationary engine, powering cotton mills and a variety of machinery; the state of boiler technology necessitated the use of low-pressure steam acting upon a vacuum in the cylinder, which required a separate condenser and an air pump. Nevertheless, as the construction of boilers improved, Watt investigated the use of high-pressure steam acting directly upon a piston, raising the possibility of a smaller engine that might be used to power a vehicle. Following his patent, Watt's employee William Murdoch produced a working model of a self-propelled steam carriage in that year. The first full-scale working railway steam locomotive was built in the United Kingdom in 1804 by Richard Trevithick, a British engineer born in Cornwall.