I suggest you try the Denver Public Library which has a mountain of good photos, especially by Otto Perry. They will sell the rights to use them or you can get prints at a nominal fee as well.
My suggestions for Colorado would be the Denver and Rio Grande Western....
Salida, Colorado
Alamosa, Colorado
There should be a few images around of those two locations, and maybe even some with the two gauge trains in it.
Sorry I don't have the url for the DPL, but someone else here might. You can go there, and type in the search "dual gauge trains", or the two towns and see what pops up.
On the Alamosa-Antonito section they ran dual gauge cars on the same train...that is narrow and standard gauge cars on the same trains.
Greg Scholl
[www.gregschollvideo.com]