Whoever you are......too bad you couldn't use your name, so that I could respectfully address you....All you need do when wanting to mount guard rails to plastic ties, is to drill appropriate sized holes and use metal spikes. If you use a pin vice and a drill size small enough, the spike can be forced into the holes with enough tightness to stay.
Bridge ties should be used on bridges and trestles. The only plastic tie strip I know of for bridges was produced by a company that doesn't market them now. So...you are stuck to use wooden ties, and hand lay your rail. Don't be afraid to do it. It isn't that hard, and gives your hard work building that bridge, or trestle, the GREAT finished look you will appreciate for years.
Fred Mills