There’s a lot of details to be considered when making a roof.  Not just the compound curves on the ends, but the ventilators as well.

The roof ventilators on standard heavyweight cars were different from the ventilation system generally in use on cars of an earlier era. In brief, the clerestory was introduced aboyt 1850 and that design lasted until the monitor roof was universally adopted by the streamlined cars in the late 1930s. Initially, windows were placed in the clerestory augmented by rooftop vents. About 1880 the Creamer ventilator was introduced and for 15 short years was the accepted standard though there were many other ventilators around. But by 1905, the advent of the Garland ventilator and its universal adoption by Pullman shortly thereafter meant that seemingly every car was Garland equipped by 1910 and remained so till air conditioning was introduced in the mid 30s. In fact Garland ventilators were so successful that clerestory windows were gradually phased out becoming extinct during the 1920s.

The Garland ventilators - 5 per side on standard 80 foot heavyweights - could do a complete air change in less than 2 minutes on a car moving 30 mph.