What is the name of the largest steam locomotive ever built?

The largest steam locomotive ever built is commonly identified as the Union Pacific Big Boy.

Built by the American Locomotive Company for Union Pacific, the Big Boy was an enormous articulated 4-8-8-4 freight locomotive. Twenty were constructed in 1941 and five more in 1944, giving the class 25 engines. They were designed to move heavy wartime-era freight over the steep Wasatch Range between Wyoming and Utah, later working over Sherman Hill.

A Big Boy measured about 132 feet (40.2 metres) from locomotive front to tender rear and could weigh more than 1.2 million pounds with its tender. Its two sets of eight driving wheels allowed the long engine to negotiate curves while producing immense pulling power. The class was designed for 80 mph, though freight service normally ran much slower.

“Largest” can depend on whether length, weight, power or another measurement is used, and the Chesapeake & Ohio Allegheny has sometimes been claimed as heavier. However, Big Boy is the standard quiz answer and is widely recognized as the largest and heaviest reciprocating steam locomotive class. Union Pacific No. 4014 returned to operation in 2019.