Hi Dan,
I prefer to use track screws whenever possible because they will never work loose and get picked up by speaker magnets. Unfortunately, there aren't a lot of good screw options for ScaleTrax. A #2 wood screw fits well in the holes Al mentioned. Commercially it's very difficult to find any #2 wood screws other than brass. Atlas sells #2 x 3/4" steel blacked track screws with a pan head and sells them as track screws for their own track. These work fine with ScaleTrax as well, but I never cared for the look of the screw heads sitting on top of the ties. When I started my current project using ScaleTrax, the NorthWest Trunk Lines, I wanted something better. I had a batch of #2 x 3/4" blackened steel flat head screws made. These require you to counter sink the top of the tie, but fit flush when installed. Here's a photo of the screws I had made.
In my mind the results were the effort.
The minimum production run was 100,000 screws. I have a few left over for anyone who's interested.