Lionel supplied an OTC contactor for a Barrel Car that was snapped to the bottom of the rails about halfway along a Barrel Loader. The rear shoe unloads the car and the front shoe loads it. As Rob posted, one 6019 or "O" Gauge UCS will work, too.
Be sure the Barrel Loader has a brown or green flap at the top of the ramp. Usually vertical, it moves beneath a barrel to place it on a Barrel Car. ALSO, a rectangular black clip on the wide platform fills the space by the track so barrels will roll from a Barrel Car on the loader.
It's best to use two different variable voltage circuits for the OTC/6019/UCS and for the Barrel Loader.
When loading the Barrel Car, turn down the Loader and operate the Barrel Car to move barrels up the ramp on the car and make room for the rest as they are loaded.
It takes practice, but a Barrel Car can vibrate on low voltage and "snug barrels up" against each other until the whole ramp is filled. Visitors are amazed because barrels are loaded with no belts, chains or moving parts. A Loader can be turned down or off while barrels are moving up the ramp on the car. And all the time there is a low, businesslike hum.
A Barrel Loader and a Barrel Car are among my favorite Lionel accessories. They are fascinating to watch. They are real crowd pleasers.