Portable camera sliders are great, but they can't replace very long tracked dolly movements. Here's a recent DIY Seated Video Track Dolly project from Vimeo member TLA Productions. If you're planning on tackling one of these projects, here's a video with some tips to make your platform more stable, and some considerations about working on uneven surfaces.
I've actually built one of these before, and the most critical part is assembling a good wheel set. The wheels really have to be drilled perfectly to sit on the track exactly the same. Not an easy thing to do when you have a 16 wheels. If you drill just a few millimeters off, you'll notice it once you're going down the track.
For DIY projects, you'll most likely be using off the shelf pipes instead of professionally laid out tracks. For this, the swiveling design wheel set is the best because it can move and correct for misaligned tracks or imperfect alignment of your wheel mounts. A good set of swivel assemblies can also be used on curved tracks. Keep in mind you'll have to dish out more for a decent set of wheels and high quality bearings. There's a few good looking assemblies on eBay (Click Here).
DIY Track Dolly Swivel Wheel Assemblies