The Big Boy is a 4-8-8-4 articulated locomotive. I thought that was as big as they got, basically two locomotives under one boiler, hinged in the middle. But Wikipedia has an article on Triplex locomotives with three sets of 8 driving wheels and 6 cylinders. The triplex locomotives were unsuccessful, and were rebuilt into 2-8-8-0 and smaller locomotives.

There were plans for Quadruplex 2-8-8-8-8-2 locomotives and even a Baldwin Quintuplex 2-10-10-10-10-10-2, but these were never built.

-Joe-