Atlas O PS-4427 Hoppers - How to Open?

I've only had to do this on one car (speaks well of the cars since I have a lot of them) and it's tricky. Make sure the hatches are secure. If you can, carefully pry, cut, or wiggle the end roof walk supports free from the roof where they meet the underside of the roof (I broke mine on the car in question). There's a fine seam between the car roof and the walls. You slip a fine knife blade (don't use an X-Acto; use a pen knife) and twist slightly and the roof pops loose. Be careful not to mar the paint and work your way around. Use Goop, Goo, or FixAll thick superglue to re-secure the weights. It has been so long, I'll re-check the cars to see if there's another place where the end supports can be more easily freed.

Thanks very much for the information, Matt. I could tell just looking at it that it was going to be a delicate job! There is a small screw holding each metal weight down (there are three, one in each bay). One of the screws broke in two of the hoppers I have. How that happened, I have no idea. The cars must have been jarred in shipment is all I can figure. 

I suppose if I'm adventuresome I could try opening the hatches, and using a magnet (the kind on the end of a rod for picking up screws) and see if I could lift up the weights, squeeze some glue down the hatch onto the bottom of the bay, and then push the weight down on top of it. I might try that first - I don't think it would hurt anything to try it.