These small signs are remnants of the Pearl District's industrial past when NW 13th Ave. had railroad tracks running down it to service the loading docks on both sides of the street.
Oregon Transfer Company Building, Warehouse No. 1, NW 13th at Glisan, 2009:
Prael, Hegele Building, NW 13th between Hoyt and Irving, 2009:
Dale Building, NW 13th between Flanders and Glisan, 2009: