Might be something in either of the two books written on this line. If not there, probably not going to find it unless you get lucky and find an old timetable somewhere.
As far as the rail size goes, why not go to Cortland and look at the remaining segment? I sincerely doubt much of the rail's been changed since it was put down. Even if it has, there could be traces of older rail buried in crossings and so forth.