I’m just a hobbyist and these are my experiences…

I’ll always defer to making reliable operation over fine detail. Here’s a neat modeling tool that I use for weighting open topped or flat cars – Liquid Gravity. The liquid really is tiny balls of weight. This allows one to place the maximum amount within a space. You can read more about it here: https://deluxematerials.co.uk/products/liquid-gravity

Liquid Gravity fed into the false compartments of wood blocking racks that are hung under a MOW flat car

Here’s how I use this:

  1. These little guys do flow like liquid, so I go carefully when pouring. I put a tray (a pie pan) under the model to catch any that get away. Then I can pour them back into the bottle after I’m done. These are rather expensive balls, so I don’t want to waste any.
  2. Since I use thin CA to glue them down, and thin CA flows like water, I try to make and seal up “compartments” in which to pour the balls in.
  3. I work in small areas, filling the sealed compartment and levelling them to a desired height. Too large an area is prone to become a messy experience.
  4. To glue them in place I just add a few drops of thin CA, but not too much as it can leak through the smallest openings or flood the compartment.
  5. Just after applying the CA, I add a few drops of CA Accelerator and that freezes them in place.
  6. Like anything in this hobby, using Liquid Gravity gets easier with practice.
MOW flat car (Tichy) with Liquid Gravity poured and secured into underbody compartments
and the racks that will also be mounted over them
Another Tichy AC&F flat car with Liquid Gravity – applied just under the side sills.

On straight side silled flat cars like the examples above, there isn’t enough space to gain the recommended weight which is 3-3/4oz for a 40′ car (the model itself will make up some of that weight of course). But the MOW flat car above weighs 2-1/2oz and the plain flat car without a load weighs about 2oz (the Tichy flat car weighs 1oz). Every bit helps.

MOW flat car with racks in place

A fish belly side sill car will allow more weight. The 240g bottle (8.5oz) should do 3 cars.