Lego Train Compatibility

I'm currently using the old 9 Volt track system for my Lego trains, but am considering switching to the new power functions motor and IR control. Does anyone know if the power functions motor will run on the old track, or will I have to buy all new track too? Thanks in advance. PS. I DO realize that I can't power the PF motor thru the 9V tracks... just wondering if they are size compatible.