Thomas trained as a mechanic at the Tredegar Ironworks, rising to become Chief Engineer there in 1828. He understood the potential offered by steam locomotives and, in 1829, supervised the first successful locomotive journey on rails in Wales since Richard Trevithick's trials at Penydarren some twenty-five years earlier.
The ‘Britannia’ steam locomotive ran on the Sirhowy Tramroad from Tredegar to Newport and heralded the steam age in south Wales, as the Monmouthshire Merlin reported in December 1829:
'It is generally believed that the engine will answer the purpose intended and that horses will be put out of request.’
Over the next 15 years, Thomas built 10 locomotive engines at Tredegar: St. David, Tredegar, Jane, Fanny, Charlotte, Lady Sail, Lord Rodney, Prince Albert, Dispatch and Bedwellty. By 1841, of the 7,324 people living in Tredegar, 2,757 were working for the Tredegar Iron Company, which had grown as a result of the skill and foresight of men like Thomas Ellis.