The invention discloses a flat grinding device for coal in train carriages. The flat grinding device comprises slideway upright columns, a grinding roller frame and a driving mechanism, wherein the slidway upright columns are arranged on two sides of a train track, and a cross beam is arranged at the upper ends of the slideway upright columns; the grinding roller frame is glidingly arranged on the slideway upright columns, and grinding rollers are arranged on the grinding roller frame; the driving mechanism drives the grinding roller frame to lift or drop along the slideway upright columns through steel wire ropes and a direction-changing pulley block, and then the grinding rollers are pressed on the coal in the train carriages. The device can be operated by a single worker, and the problem of shortfall of coal transportation is solved.