gradient post
noun1
A post or stake indicating by its height or by marks on it the grade of a railroad, highway, embankment, etc. at that point.
- Long before Shap platform showed up around a corner and the two arms on the gradient post drooped in both directions at once, Duchess of Buccleuch's amiable throbbing purr at the stack [funnel, chimney] had become a fierce freight-engine bark, as she resolutely dragged at her enormous load.1949 January and February, F. G. Roe, “I Saw Three Englands–1”, in Railway Magazine, page 12: