It probably should've been obvious, but I've just found out the hard way of the effects of drying plaster (Woodland Scenics Plaster Cloth) and Kato Unitrack when kept in a small enclosed space.
I've been building a new layout recently in a coffee table I acquired from a secondhand store. It's around 80x80cm and fits a nice loop-the-loop-the-loop layout.
After planning and purchasing the track, I started to build up the scenery. Once the foam was down, the plaster was laid. Due to wanting to be neat, I returned the setting/drying scenery back into the table each night to dry. I noticed on the second day that the plaster hadn't really dried that much, so I chose not to run any locos (there had been a C50 steamer sitting in the table overnight too!.)
The next day I then noticed that the track had a strange tinge to it. I attempted to run the steamer, but the performance was terrible (although this was second hand and hadn't run 100% in the first place.) Another locomotive didn't do much better.
The cause was obvious on closer inspection... (First image: notice that left track is shiny and right is dull. Second image: see mould.)
After 'phoning a friend' on the JNS Forums, a few possible options came up... the most probably one being the chemicals used in the plaster cloth. These could well have hung around inside the glass table whilst the plaster was drying and adversely affected the track.
I'm still in the process of cleaning it all up (a quick once-over with a cleaning block worked fine)... the sides of the rails are still tarnished.
I'll update again here once I've worked out the best/quickest/easiest solution.