I did an experiment a while back and found that despite what some may claim, solid foam has only a minimal effect on reducing noise. People might claim otherwise but I actually used a decibel meter in my experiments and the difference was minimal. All the foam did was change the tone of the noise (admittedly, this itself was an improvement). If you think about it, it makes sense that pink foam wouldn't work. Sound reflects off smooth, solid surfaces and pink foam is very smooth and quite solid. Carpet/fake grass is a much better way to go and your best bet for silencing FasTrack. You can also fill the cavities in the FasTrack but this is pretty time consuming and still doesn't address the hollow rails of FasTrack. If noise is a real concern go with Atlas.
Finally, if all you want to do is keep the tracks from wiggling apart - a real problem with some FasTrack (especially the O31 curves) - a far easier and cheaper solution is round wooden toothpicks. I did this on my layout and it keeps the track from wiggling apart over time.
