A small clarification:

B&O long stock cars were a 1960-1962 vintage innovation.  Not steam era.

By the late 1950's, B&O was returning their leased wood Mather stock cars.    During 1960-62, B&O leased some converted ex-ATSF whalebelly boxcars from Mather sucessor North American, these were dubbed "Livestock Special" in a colorful paint scheme.  In 1962, B&O concocted its own 50' stock cars by extending M-26 subclass cores by 10 feet.


B&O 40' Mather stock car link (scroll halfway down): http://www.richyodermodels.com/mather-stock-car-reserve.htm

B&O ex-ATSF Livestock Special stock car link:  http://rrpicturearchives.net/showPicture.aspx?id=4604645

B&O S1/S2 extended stock car link:  http://rr-fallenflags.org/bo/bo113392jpa.jpg