I have some perfectly straight 1 x 4's that I use for straight edges. One each at 4' long, 6' long, and 8' long. I secure them to the work with a C-clamp on each end, then further screw them down tight with deck screws at 1' on center, so they can't move. Then slide my Skill Saw down the straight edge to do the cutting. Yeah, it leaves a screw hole every one foot, but in most cases it doesn't matter for my purposes. I do this outside of course, just because I don't want to make a saw dust mess in the house.
The Menard's Master Force Track Saw looks pretty cool, however, even if limited to the nominal 4' of track. For the price, I may have to go check it out. I'd best wait until all the train stuff is put away for the season, so that the saw doesn't end up costing me a whole bunch more money should I decide to get one. 
