I, too, use speaker wire for the buss.
Dave
Lackawanna Route of the Phoebe Snow
Stranded speaker wire from Wal-mart, K-mart, Home Depot, Lowe's, etc. works just fine. The speaker wires sold by nearly all of these stores is Philips brand, and comes in two primary guages: 18 and 26. Eighteen gauge is heavier wire and would be plenty sufficient to handle the current output of a Digitrax system.
I wired a large HO scale club layout using this wire and an NCE PowerPro 5 Amp system and boosters, and we have had no voltage drop problems anywhere on the layout.
It really depends on the layout size and how far the wire has to go. 14awg stranded is about the largest wire that will fit in the Digitrax plug without doing damage. Stranded works better with the screw terminal because it compresses better than solid.
Martin Myers