I shot this one last week sitting in siding at Highfield on the CSX Hanover Subdivision at interchange with Maryland Midland. 20151106_105336 by Adam Henry, on Flickr
Most definitely cement. The car is now marked for Lehigh Cement. They have a plant in Union Bridge, MD where MMID is based. Lehigh actually has ownership in the railroad. MMID also moves lumber products and stone aggregate but probably more than 80% of traffic is cement. This car was in a string of about 20 waiting for CSX to pick up.