All of my open-topped terminal service trains (coal, ash, sand) feature removable loads.
In the photo above, the two lower items are loads for hoppers. They are made with a tray made from .040″ sheet styrene with .125″ x .125″ and .060″ x .125″ strip styrene stiffeners. The trays are smaller than the opening in the car by about 1/16″ per side. This does leave a gap when installed in the car, but not too noticeable in my view. I opted to go for reliable function over scale appearance. Once built up, I “CA” four 2″ steel nails in the centre of the tray (they are nested side-by-side and flat on the sheet). Then, I spray the assembly flat black. The edge of the tray is lined with 6mm Tamiya Tape all around. The coal is sprinkled inside on top of water-diluted white glue brushed on the tray. As more coal is added, I then applied Woodland Scenics Scenic Cement liberally with an eyedropper. The other two loads shown are ash, with one as a partial to add variety to the spotted gondola on the ash track.
A telescoping magnetic wand is used to remove the loads. It came with a rubber cap and helps to reduce the attraction; in case this needs to be further reduced, a disc of styrene and be added inside the cap in front of the magnet.
The wand is simply spotted over the centre of the load and is lifted out and away.