It may not be "breaking a sweat", but it's pulling with a really small motor. The Lionel Legacy K4 is based on the K-Line design, and I've had a bunch of them across the bench for motor and gear issues.
Have you had any other gearbox issues with Legacy steamers produced in the last few years, on the bench?
Lionel has adapted that fragile gearbox to just about every model. Light and heavy Mikes, N&W J's, NYC Mohawks, the H10's, heavy pacifics, the new northerns, the new 2-6-6t, and at least one of the driver sets of the Legacy mallets. Heck, it might be in everything by now.
As I mentioned some time ago, I had a first hand experience with the secondary gear bushing beginning to fail in my Legacy K4. Even though well lubed, it actually started to produce metal shavings.
It takes a ton of grease in the gearbox screw port to get it transferred up to the worm with this design, and most folks aren't aware that theres an extra set of bushings in the chassis above the powered axle that can only be lubed through the spokes.
I only own two models with this design, and after the K4 failure will never purchase another. I suspect, eventually we'll see more failures of these over the next several years as folks put some miles on them.
I wonder what the purpose of the design is? To attain a slow speed that can't be reached by the electronics alone? I know I won't have a gearbox failure in my old lurchy TMCC steamers.