milepost

A post on a highway, often with one or more fingerposts, showing the distance in miles to nearby places.

A sign or post beside a railway marking the distance from the (actual or nominal) start of a line (usually the principal terminus or junction with a more major line).

But beyond these cut-offs, to avoid the wholesale alteration of all mileposting and mileages—of bridges and culverts, for example—the original mileposts have remained unaltered.

[...] the 4 miles at 1 in 180 up to Sanquhar were mounted with no greater fall in speed than from 65 to 59 m.p.h., after which, possibly as a result of easing the engine or because the strain on steam supply was beginning to tell, the final 3½ miles up at 1 in 200 up to milepost 59½ were surmounted at a minimum of 49½ m.p.h.

To place mileposts along (a road, etc.).