Most flatcars (flat wagons) cannot carry more than one standard container on top of another because of limited vertical clearance, even though they usually can carry the weight of two. Carrying half the possible weight is inefficient. However, if the rail line has been built with sufficient vertical clearance, a double-stack car can accept a container and still leave enough clearance for another container on top. This usually precludes operation of double-stacked wagons on lines with overhead electric wiring. China runs double-stack trains with overhead wiring, but does not allow two maximum height containers to be stacked.