> could somone tell me what [sell
> them on the rail means]?
You get paid for the carcass that hangs on the rail, not the pounds of live animal you deliver. You get paid for pounds of carcass and will get a discounted price for a poor quality carcass. You may or may not get premiums for a high quality carcass.
Selling under an array of premiums and discounts for carcass quality and yield grade is called "on the grid"