K-Line Big Boy 4015S - 2nd set of drive wheels don't turn

First step is to closely examine the drive rods on that set of drive wheels.  Top suspect is a drive rod binding.  Many times one of the rod screws works out and then catches the drive rod as it tries to go past.

Thanks for your quick response @gunrunnerjohn! I've been lurking on the forum over the last few weeks and have consistently seen you providing solid advice and sharing what is obviously a wealth of knowledge, so I was hoping you'd respond! 😁

I took a closer look at the drive rods as you suggested and didn't notice any differences when comparing the 2nd set that wasn't working with the first that was. After that, I figured I'd throw it on the tracks again and see what happens. I had forgotten to reconnect my Legacy base, so when I started cranking the voltage, the Big Boy took off like a rocket! Thankfully, I caught it before it was able to execute any crazy maneuvers. I reconnected Legacy base, started it up, and it ran beautifully! I'm not sure if things just needed to loosen up, and maybe the reverse direction test last night + the quick 0-60 this morning took care of that, but I'm quite happy my favorite (and only) steam locomotive is back up and running!

Here's a quick video of the Big Boy pulling my string of wood side reefers and matching caboose - just FasTrack on the floor right now, but I'm already having a great time just watching the trains roll by like I did when I was a kid 😊

I have this same locomotive, and love it.  I've also been in it a couple times.  I would make sure that the screw on the bottom of the driver set is tight, and well as take the boiler off and make sure the motor bracket screws are tight and the motor is properly engaging with the axles.  Just a little off and it'll bid and or chew up the gears.  Mine has so many hours on it the gears are visibly worn.

I'm a bit of a lubrication nut and would do a thorough lub of the rods and axles.  On my Big Boy, there are removable plates on each set of drive wheels from the bottom with the engine on its back; easy to get at the drive gear to clean it and add lube (I use RednTacky on the drive gear); however, I will defer to Sinclair on the way to get at your drive gears.  Even when I get a new Lionel loco, I give it a thorough lube - oil rods and axles and grease gears.

You will find a great bunch of guys on this forum for any help you need; the fellas helped me repair my Big Boy when the main smoke unit failed even right down to how to solder.