According to the excellent series of Larry DeYoung's "EL in Color" books, the highest # "JC xx" MP ever probably would have been Dunkirk, NY, something like "JC 459" (Salamanca would have been "JC 412" (I think), and Dunkirk was 47 miles further west). So perhaps the highest # JC milepost might still be at Salamanca since I think the Salamanca to Dunkirk branch has been abandoned.
Salamanca was the junction of the original Erie route Piermont to Dunkirk and the Atlantic & Great Western, which ran Salamanca west to Dayton, OH. West of Salamanca, Erie mileposts were "S xx" with mileage starting at Salamanca, and going to "S 388" at Dayton.
Mainline milepost mileage restarted again west of Marion to Hammond, originally the Chicago and Atlantic RR. I'm guessing the highest MP west of Marion was "M 249" at Hammond?