Mill Yard is short for the former Rolling Mill Yard which is between Besly Court and the UP Northwest Line, west of Sipi Metals, where Wabansia Avenue dead-ends at Besly.
At one time US Steel (formerly Illinois Steel) had a plant on the west bank of the North Branch of the Chicago River that was serviced by an isolated EJ&E operation. (US Steel owned the EJ&E until its recent sale to CN). This in-house EJ&E operation of US Steel interchanged with the Milwaukee Road via track that ran west in Wabansia to the Rolling Mill Yard.
In later decades Proctor & Gamble took over the former US Steel site and now a City of Chicago maintenance facility is in its place where Wabansia ends on the eastern side of Elston.
Some of the tracks that once reached US Steel and later Proctor & Gamble are still embedded in Wabansia.