Employee was riding in Alpha 14 man diesel mantrip facing to the side (perpendicular) to the direction of travel. He had his feet propped upon the brace. When the mantrip made a 90 deg turn to the rt, the trip was hitting the coal rib. His lt foot hit the coal rib first, he was trying to move his lt foot when toe of his rt foot caught between frame of mantrip and coal rib.