I built our pack's track last year and used Power Pro fishing line. Power Pro is a high-tech "super-braid" line that has almost zero stretch and is considably thinner for a given pound test. It's surface is also very slick. The 60 lb test I used is about the same diameter as typical 20 lb mono.
I tested it in the back yard before deciding to use it and my test rocket required about 30% fewer turns to go the 50 feet verses regular Mono and got there quicker too.
This is fast stuff and I highly recomend it to anyone wishing to tune up their track. The no-stretch property of this line makes it easier to get each lane set to the same tension also.