I've never used either felt or indoor/outdoor carpet. It seems to me the carpet might deaden sound better. Is that correct?
Carpet padding covered my last layout, with electricians' wire ties securing the track, and it was good at sound deadening, and could easily be painted black for roads. Another great thing was that Marx telephone poles with those wide plastic bases could be simply installed by slitting the padding and sliding the base under. Great when you bumped into poles too. The pole wouldn't break, just tilt. However, the padding was a pain to drill through: The material would wind around the bits.