I just had to Google this, coal hoppers, and sure enough, there are some interior shots, kind of hard to make out, and the pictures are usually part of a thread on weathering the inside of empty hoppers.
It looks like the seam has something to do with the structure inside the car, that makes the product flow out the bottom.
https://www.google.com/search?q=coal+hoppers&hl=en&tbm=isch&tbs=rimg:Cf_1OwU8zophIIjgI9F3oVR-CcD6QCyBj4OvZEYzjcCUFfZLwjqnyRKt6ng0HxchO-RScuhESaZTvwj7DtlpHuZKtgyoSCQj0XehVH4JwEd_1kkx_1pb4unKhIJPpALIGPg69kR3-STH-lvi6cqEgkRjONwJQV9khGCypkjn9MBwioSCfCOqfJEq3qeEbpZN0mydRzbKhIJDQfFyE75FJwRjh_1hTcBKl2wqEgm6ERJplO_1CPhGNR-EwzPLD4yoSCcO2Wke5kq2DEdTTT41NoblK&tbo=u&sa=X&ved=2ahUKEwjskPK13oXaAhWJu1MKHbybCzsQ9C96BAgAEBg&biw=1263&bih=558&dpr=1.25#imgrc=_
Now THAT is a long link!
Mike.
My You Tube
It happens with covered hoppers, too. In particular FMC 4700's:
Ed
Looks to me like it is just a seam. There is probably a bumpy weld along the inside. The stanchions may be spot welded. Like many things, steel is sold in standard sizes, so it is cheaper if you can order it that way.
Brent
"All of the world's problems are the result of the difference between how we think and how the world works."
I've been looking at webcams to understand how to weather coal cars. I noticed a horizontal line on these coal cars and another on the ballast cars. Is this related to the construction and the inside of the cars?
opps Edit
Henry
COB Potomac & Northern
Shenandoah Valley