The term 'Iron Horse' is a nickname for a TRAIN, which is a 5-letter word. It is also known as a steam locomotive, which is a 15-letter word. The term 'Iron Horse' was first used in the 19th century to refer to the steam locomotives that were used to power trains. These locomotives were made of iron and steel, and they were powerful enough to pull heavy loads. The term has since become a popular nickname for trains, and it is still used today.