The American Car & Foundry ‘ACF’ 3500 cu.ft. covered hopper, built between August 1964 and June 1966 was an early innovator in railcar pressure differential unloading. A marketing moniker of these hoppers termed "Flexi Flo" coined by the car's biggest customer, the New York Central Railroad, gave these cars a permanent nickname which lasted their lifespan of over 50 years. Despite their prevalence in the Northeast, these cars were seen throughout the entire US and Canada by virtue of transfers, subsidiaries and sale to other shippers. New paint schemes and a wide variety of patchouts allowed many of these cars to last into the mid 2010s.

Rapido's models features three body styles & two top hatch sizes, 100-Ton & 125-Ton Barber S-2 trucks, etched metal running boards, both wire and injection-molded grab irons, fully detailed underbody with full discharge piping, brake equipment and piping variations, factory-installed air pipes and coupler cut levers and metal Macdonald-Cartier knuckle couplers. Road numbers #897684, 897690, 897712, 897761, 897770 and 897794.