On February 21, 1804, British mining engineer, inventor and explorer Richard Trevithick debuted the first full-scale working railway steam locomotive in the Welsh mining town of Merthyr Tydfil. Following that debut, locomotives have been powered by a myriad of fuels, including wood, coal and oil.

Historically, "old" trains were primarily powered by steam engines, which dominated the rail landscape from the early 1800s until the mid-20th century. These locomotives functioned by burning a fuel source—initially wood, then later coal or heavy oil—to heat water in a large boiler. The resulting high-pressure steam was piped into cylinders where it pushed pistons back and forth, turning the wheels via a system of rods and cranks. In the late 19th and early 20th centuries, some early "old" trains also utilized compressed air or even static steam (fireless locomotives) for industrial use where sparks were dangerous. By the 1930s, the transition toward diesel-electric and electric power began, which eventually replaced steam due to their superior efficiency and lower maintenance requirements. In 2026, while steam trains are almost entirely relegated to heritage and tourist lines, they remain a marvel of mechanical engineering, representing the era that first connected continents and powered the Industrial Revolution.