MoPac built these cars not only to replace aging heavyweight mail storage cars, but to take advantage of Post Office contracts that paid the railroads by the linear-foot of mail hauled, not weight or volume. Hence, these cars with 8-foot interior height earned as much revenue as a car with a 10-foot interior (such as the ACF “economy” storage cars). The cars were also built by MoPac forces and were less expensive than cars from ACF or Pullman-Standard. Had not the Post Office pulled mail contracts from the railroads in 1968, this type of car may have seen wider use by other railroads.

DR says: ” MoPac built these cars not only to replace aging heavyweight mail storage cars, but to take advantage of Post Office contracts that paid the railroads by the linear-foot of mail hauled, not weight or volume”

Maybe they should have hauled mail in covered gondolas to make even more money!

The 70′ lightweight mail car was also instantly recognizable. It’s very easy to spot those cars in a train consist whether on the MoPac or a connecting road. You don’t need to see the letterboard to know it’s a MoPac car.

Re the underbody detail on these cars —

The diagram says the brake gear is AB-1B. I recall a post from Dennis Storzek on STMFC or somewhere on exactly what it is. From memory, it’s basically the same as a freight car AB brake with a few enhancements that might not be visible from trackside.

And of course, there’s the steam line, which was wrapped with asbestos insulation. I’m not positive about the line size or the insulation thickness, but I believe you’d end up with an effective 4″ diameter or so line under the car. MoPac had the steam line running close up under the floor from each end through the bolster, then dipping down a few feet past the truck so that it cleared the crossbearers. And of course the steam connection was on the left side of the coupler at each end, so it also crossed over the centersill. As such, it’s pretty visible under these cars.

RG7