To expand on what John said, the 1918 valuation map serves as the basis for trying to figure things out. There are enough things still present in the yard (old telegraph poles or stubs, the boxcar sheds, the roundhouse itself and the occasional tie in its original location) to provide reference points using old photographs. One of the long term volunteers, Jeff Ramsey, has spent quite a while studying the track layout and can pretty much tell where each track should be. as such, he is the "official" surveyor. Also, though the Como yards are essentially a barren, cinder covered wasteland, at times it is still quite easy to see where tacks went, if you know where to look. The grass still grows a little fuller in the spaces in between where the ties once were and after a light rain, the difference in the color of the ground can show a pattern of where ties once were. (Thanks to Jeff for pointing this out to me).
The Gunnison main line was the easiest to locate as it was built before the yards and runs in a very shallow cut through them (as seen in rehunn's photo in the post above)
Jason Midyette