It's so long,during initial assembly they had to hinge it's frame to accommodate the corners..LOL
The design was patented by Anatole Mallet (pronounced "mal-lay") in 1884. Mallet's design, however, was for a compound, articulated locomotive (steam from the rear set of high-pressure cylinders was re-used in the front set of low-pressure cylinders before being exhausted to atmosphere). The Big Boy is a simple articulated locomotive (all four cylinders are of the same diameter, and receive steam from the boiler at the same time). You are correct that the frame is "hinged" in the center to allow the locomotive to negotiate tight radius curves. The rear half of the frame (the rear "engine")is attached to the boiler at several points (some of those attachment points are sliding surfaces to allow for the expansion and contraction of the boiler), while the front half of the frame (the front "engine") is allowed to swivel under the boiler (a large, flat bearing surface transfers part of the boiler weight down to the frame and thus the wheels).
Many railroads in the U.S. operated articulated (both compound and simple) locomotives of various sizes and wheel arrangements in both freight and passenger train service. The Union Pacific's "Challenger" (4-6-6-4) locomotives and "Wasatch" (4-8-8-4, aka "Big Boys") were among the largest and most famous, but there were larger (heavier) examples constructed for other railroads.
There are currently two examples of compound articulated locomotives running on the non-profit Niles Canyon Railway in Fremont, California, and a third running on the Black Hills Central Railroad in South Dakota. A number of other examples operate in various European countries, too.
Fascinating machines, and a lot of fun to run.