I stumbled across this post (and joined the forums) looking for the standard width for dolly track.
I would add to the discussion: use galvanized steel pipe for your rails (8ft. pipe used for chain link fence). It's a bit heavier than aluminum, but it's much more sturdy (doesn't ding and bend) and it won't rust. You don't get the mess of oiling the track, and you don't need to mess with the considerable upkeep of painting.
Here is a link to my blog post on building a very affordable dolly with track.
Cheers.
