James Watt invented the steam engine after he realised the power of enclosed steam. George Stephenson later found a way to harness the steam in such a way that its power helped rotate wheels. Thus the railways engine took birth.

The first person to harness the power of steam was the Greek scientist Heron of Alexandria in the begining of the last century.
He developed several devices that were operated by water, steam, or compressed air, including a fountain, a fire engine, and the steam engine.
The steam engine was significantly improved by Thomas Newcomen who created a machine that used steam to pump water.
And this was later further improved on by James Watt.

James Watt invented the steam engine and later Stevenson modified it .

The answer is correct. Howeveer, the idea that steam has a force struck him when he noticed that the lid on a pot water kept on the stove began to dance as the pressure of steam developed and when it becamd unbearable it just flew away.Many of us might have witnessed the same phenomenon but it was Sir James Watt who rationalised it.

However, it is said that the steam engine had been developed inthe times of the Greeks. However, as the locomotive power of steam was of no practical use in those time when transport was in the rudimentary stage yet the 'railway line'was laid on a dining t able which was long enough and the eatablle inthe cattles travelled from one end of the table to the other and then vice versa.So Sir James Watt was not necessarily the first to appreciate the motive power of steam, though this reinvention came at an appropriate time when long distances had tro be covered for travel and taking goods from places to places.