from The Century Dictionary.
- noun A bottom, as of a car, which can be let fall or opened down-ward: a common device for unloading certain kinds of railroad-cars.
Sorry, no etymologies found.
All I got out of it was where to find an ashpan drop-bottom, steam accumulators and why they are still used, and the ability to recognize a three-cylinder locomotive by sound alone. '
The stone was shipped in drop-bottom cars and dumped into bins built on the ground under the trestle.
The wagons were patent dump wagons of the drop-bottom type.