Thomas is a small blue tank engine.
He liked playing tricks on the big engines, especially Gordon. He often takes becomes concieted, like when he crashed into the stationmaster's home.
Thomas is cheeky, but is good at heart and generally behaves well. He loves teasing the others and, on occasion, brags about his superiority, but is always brought down to earth in due course.
Although their friendship has been turbulent at times, Percy is generally portrayed as Thomas' best friend, especially in the television series. As well, Rosie idolizes him.
Thomas is based on the Billington E2-Class 0-6-0T locomotives built for the London, Brighton and South Coast Railway between 1913 and 1916. He is apparently one of the last batch built as only these engines featured his distinctive extended-side-tanks, projecting forward over the leading wheels and little downsweeps at each end of footplate. The E2s were introduced in 1915 to replace earlier members of the E1 class, which had been scrapped. The last five in the ten strong class had extended side tanks to give a greater water capacity, but all engines had two inside cylinders and driving wheels 4'6" in diameter.
After so many years of service, all of the Billington E2s, both original and modified, spent their last working years at Southampton Docks and they were all scrapped by 1963; the very last pair being Nos 31204 and 31209, the latter being amongst the last batch. Rosie's class replaced them.
Thomas has a slight modification of having wheel splashers over his front wheels whereas the original E2s did not.
Thomas is painted in the North Western Railway's standard blue livery with red-and-yellow lining and the number "1" painted on his tank sides in yellow with a red border.