I recently tried a DiGitrax DZ 125 in a SW8. It doesn’t fit. The decoder is too wide. I did see a guy post photos online using a TCS Z2. My install http://www.trainboard.com/grapevine/blog.php?b=2126. Greg
Not impossible but installation does require some work. There are a couple of ways to install. Simp;est is to leave out the cab weight and install a small decoder in the cab. Harder is to cut some of the frame up under the front hood. You get to keep the cab weight with the latter.
Here’s a link to just about every type of installation in LifeLike SW7,9,1200. Same steps for the SW8.