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 😊