As I was "dusting" (hah) this AM, I decided to move my displayed MTH #1 scale NYC J3a Hudson to a better position. Picked it up and found 2 big puddles of dark grease on the table top (I have no track under it) and all over the bottom of the #3 driver tires and flanges.
Great, a leaking gearbox. I cleaned it up (409; lighter fluid) from the furniture and flanges (seems awfully thick for grease...), and then noticed that the gearbox was on the #2, not the #3, driver axle.
Then I realized that it was the traction tires that had melted into 2 big puddles - seriously, a viscous liquid - in my living room. I have never seen this on any locomotive. The loco has never been run by me, though it was lightly used when I got it. My house is cooled in the summer, heated in the winter, and spring/fall has the windows open - like now. Interesting chemical process, but not a temperature one.
Just a caution. I have never seen any of my O-scale MTH locos do this, but...
Note the gearbox on the #2 driver, at left. I wish that I had taken a photo of the living room table puddles and the #3 driver before I cleaned up.
Those rail slider pickups look much worse than 3-rail center pickups - but at least they are not absolutely necessary and are removable.
Couldn't help it - the bottom loco is Lionel's O-scale J3a. The boxcar is from my teen HO days (a few months, really) and I kept it for sentimental reasons. Got to re-attach a truck - broke it today, too.