As cabs grew from small shelters to larger wooden, then iron and steel rooms, the need for unrestricted vision became obvious. Windows were added, as well as doors (with windows) in the front of the cab on either side. Glass was added to the windows, and even fancy amenities like sashes and curtains, depending on the railroad and the engineer. When boilers became enormous (and since running boards are not safe to traverse while in motion), cab doors disappeared again.