SCRANTON, LACKAWANNA CO. (WOLF) — Big Boy 4014 arrives in NEPA on June 13. Each day until then, we’re sharing a fascinating fact about the world’s largest operating steam locomotive.
The driving wheels on Big Boy 4014 measure 68 inches in diameter — nearly six feet tall.
Standing beside one in person is often the moment visitors realize just how massive the locomotive truly is. Many adults are surprised to discover the tops of the wheels reach well above their waists.
The size of the wheels wasn’t just for appearance. Locomotive designers carefully selected wheel diameter to balance power and speed. Freight locomotives typically used smaller wheels for greater pulling power, while passenger locomotives often featured much larger wheels for higher speeds.
Big Boy’s 68-inch drivers represented a compromise between the two. They were large enough to allow the locomotive to maintain speeds approaching 80 mph when necessary, yet small enough to generate the tremendous tractive effort needed to haul thousands of tons over mountain grades.
There are 16 driving wheels beneath Big Boy — eight on each engine unit. Together, they support much of the locomotive’s enormous weight and transfer more than 135,000 pounds of tractive effort to the rails.
Railfans often notice another unique detail: the tires on the driving wheels are made of steel and are only a few inches wide. Despite supporting a locomotive weighing more than a million pounds, all of that weight is concentrated on surprisingly small contact patches where the wheels meet the rails.
When Big Boy is moving, each of those giant drivers completes hundreds of revolutions per mile. During its original career, 4014 traveled more than one million miles, meaning those wheels turned millions upon millions of times while crossing the American West.
It’s a reminder that some of the most impressive engineering on Big Boy isn’t just its size — it’s the fact that those wheels reliably carried one of the largest locomotives ever built across mountains, deserts and plains for decades.