I was using one of my 0-8-0's to test tracks as I repowered them. It ran just fine when I packed it up in New Jersey, but it was stuttering and stalling constantly when I put it back on the track. I cleaned the track and that didn't help very much, so I cleaned the wheels and tender pickup contacts, and the loco ran much better!
Shortly after that I put most of the locos back on the layout. Several of the were not performing up to par, running jerkily or just seeming to move reluctantly, as if it was taking a lot of power to get them to move. Finally, I turned the layout off and disgustedly headed upstairs for the night. I really wondered for a few hours why I was bothering to even try to build another layout.
The "railroad despair" didn't last long, though. Clearly all my locos are past due for some maintenance. A thorough mechanism cleaning and re-lubing is in order. Everything ran like a watch right out of the box - I only had to fiddle a little bit with one or two locos when I first purchased them. I was fortunate not to have to do much tuning-up on any of them. It's been about twelve years since I purchased the newer ones, and some are pushing 20 years old, so it's time for some rehabilitation, I think. I'll do that one at a time over the next six months or so. I've actually come around to the point that I'm looking forward to it!