Well all the links I just dug up and posted here evaporated when I hit "post"... >
I think it is a form of Russia Iron. Mostly the earlier the photo the more silvery it looks, those 2-8-0's are all taken before 1941. I can find photos of the same engine, one will look black and one will look silver (in a black & white photo) It could have a lot to due with the lighting and if the boiler was wiped down with oil it will reflect and look shiny.
The exact color is debated, but here is a color photo:
http://www.magiclantern.org.uk/images/bt3377b.jpg