Add freight train load/unload mode: through load

src/ground_vehicle.hpp

@@ -418,6 +418,8 @@ protected:
 			}
 		}
 
+		if (this->current_order.IsType(OT_LOADING_ADVANCE)) tempmax = min(tempmax, 15);
+
 		if (this->cur_speed > max_speed) {
 			tempmax = max(this->cur_speed - (this->cur_speed / 10) - 1, max_speed);
 		}