This past summer four of us did a 4 day trip on Le P'tit Train Du Nord in Quebec. We all used are road bikes, me on my Seven Alaris Ti. We all used 25mm tires. The 125 mile long trail was about 1/3 paved and the rest hard packed sand and gravel. We had no problems (other than slower going) on the hard pack, not even in the deluge we got caught in on our second day of riding.