Logging in Colorado...yes, there was some.
There were a small number of lumber mills located in the north end of the state, along the line that dropped down from Laramie, Wyoming (originally built by a railroad called something like the the Laramie, Hans Peak & Pacific (I know that's wrong, but I can't remember the proper name right now)...at any rate, this line was picked up by the Union Pacific and was their Coalmont branch for a while, up until it was sold to the Wyoming/Colorado Railroad around 1986. Wy/Co has since abandoned this entire line. They did haul lumber and woodchips up until the end of operations.
There was another sawmill farther east in Saratoga, WY that relied mostly on logs brought north from Colorado. I am not aware of any logging railroads built off of either the line to Coalmont or from Saratoga.
By far the highest number of lumber mills in the state that I am aware of were down at the southern end of the state. There were a couple mills on the D&RGW line that went west from Alamosa to Creede...one of them (at South Fork) remained in operation through the mid-1990's before closing.
There were a number of logging railroads off of the D&RGW narrow gauge line that went from Antonito westward through Chama, NM to Durango. However, these were almost all narrow gauge (if not all narrow gauge). New Mexico Lumber Company was one of the biggest.
For more information on this subject, you definately want to get your hands on a copy of the old book Logging Along the Rio Grande. Copies for sale are hard to find and tend to be expensive, but perhaps you could locate one through interlibrary loan. My favorite source for out of print books is:
http://www.abebooks.com
A check of that source turns up three available copies of this book, ranging in price from $96.78 to $195.
Otherwise, you might want to contact any historical societies in Colorado with your questions, or see if there is any such societies out there about the Colorado Midland.
However, as sumpter250 says, if you want to freelance some lumber on a railroad set in Colorado, then by all means you should do so. As noted there were a number of real operations in the southern and southwestern parts of the state, plus some smaller operations at the northern end of the state. Depending on where you choose to locate your operation, you could also have your railroad doing good business serving mining industries (just about anywhere in western Colorado) and/or agricultural traffic (San Luis Valley is an extremely productive area).
Summit
Elko, NV