Standard gauge trains

What causes wheels to swell on standard gauge locomotives? Any help would be greatly appreciated.

ZINC PEST

Thank you

From the Wikipedia:

http://en.wikipedia.org/wiki/Zinc_pest

Interesting they say that post 1960 parts are usually free from the ‘pest’. Sad to say I’ve encountered some McCoy castings from the 1980’s and '90s that have succomed to the ‘pest’ as well.

Thank you all for your replies.