Your progress along the trail is marked by literal milestones, engraved in Roman numerals of a sort.
These Roman numerals run from top to bottom. The numeral I lies on its side and V stands upside down. X is the same any way you look at it. There are no prefixes: four is IIII, not IV.
Westbound distances start from Alewife Station in Cambridge, where the trail entrance has no distance marking, and end at Bedford Depot Park with milepost X. Eastbound distances start with mile I at the Depot, so the two sides of each post always add up to XI.