This tiny little 4-wheeled, single-trucked shop switcher was used to push and pull the big electric units (like THIS ONE) in and out of the Deer Lodge, MT roundhouse. The roundhouse itself wasn't electrified so this little guy did the hard work of getting them in and out.
If you want to see a shot of this little guy when he was still in operation, check out this shot on deviantART by classictrains: www.deviantart.com/deviation/58867245/
Canon EOS 350D Digital Rebel XT
Canon EFS 18-55mm lens