RLW,
As with you, I put my entire track layout on my table without screwing it down, just to test how the engines would run on it. I had several trouble spots with certain engines. And I could not figure out why.
But, when I finally screwed all of the sections down nice and even, almost all of those problems disappeared.
After that, I had a large curved run of 031 track, that was brand new track. K-Line. All of my engines ran that curve just fine.
But, I had one Hudson Steamer that would very severely slow down and balk on that particular curve, not matter what I did.
I found out that brand new track has a chemical coating on it of some sort, and you have to thoroughly clean it off with alcohol or mineral spirits, or you will have running problems. I did that, and the steamer increased its speed, but only by about a third.
Then I took a really careful look at the steamer, and found that one of the connecting rods was very slightly bent inwards, so that on certain curves, where that rod was on the inside of the curve, the rod was rubbing against another rod. I replaced that rod, and the steamer ran about 2/3rds faster, but still not all of the way to full speed.
Finally, I just pulled that section of track, and replaced it with thoroughly cleaned, gently used, NY Lionel track.
The problem totally disappeared.
I guess my point is, that you can't just assume it is "one" factor that is causing your problem. You have to start at square one and work your way through all possibilities. The folks on this board gave me several ideas about what could be wrong, and I found that is was a group of three of the problems.
Also, I had an Lionel 022 automatic switch that was constantly sparking when locos went over it. And sometimes, it would just freeze up and not close all of the way. I took that thing apart and cleaned and examined it many times, putting it back on the track over and over. But, the problem continued.
Last week, it really started sparking bad, and I thought, Well, maybe there is just a stray scrap of copper wire stuck underneath one of the rails? I got down low with a bright flashlight, and found that there was a stray steel connecting pin, that had somehow fallen on the turn out, and gotten stuck underneath the triangular metal frog. It too me 20 minutes to fish it out with some small dental picks. I finally got it out, and the problem totally disappeared.
Is it possible that you cut some railing on your layout, and the metal dust has gotten up inside of your steamer? Or maybe, a single fleck of copper wire from one of your wire cuts?
Hope this info may help.
Mannyrock