Drill out a hole in the center bottom to take a small fishing weight known as split shot. You want to ACC the split shot in place so it does not move.
On some tank cars the ends are removeable. On others there is a horizontal seam running the length of the car. No tank car has a one-piece tank unless it was glued together.
You can also use sand, which could be mixed with diluted glue, and injected with a syringe (sans needle, and maybe drilled out a little) into a small hole in the bottom of the tank. Tape over the hole and turn upright 'til the glue dries.
The old Intermountain kits were nice for this task. You could add weight to the bottom half of the tank, then easily glue on the top portion.
Ken - an early congrats on your 50th year. The above quote would make me believe that you are talking about the days when IM actually announced, manufactured, and released for sale railroad products.
