The original choice of 8-foot (2.44 m) height for ISO containers was made in part to suit a large proportion of railway tunnels, though some had to be modified. The current standard is eight feet six inches (2.59 m) high. With the arrival of even taller hi-cube containers at nine feet six inches (2.90 m) and double stacking rail cars, further enlargement of the rail loading gauge is proving necessary.[37]