Thanx for all the suggestions.
Looks like the ends pop off but there are rails glued to them. We'll have to be real careful!
We have 2 Atlas tank cars and they are both 1.2 ounces under weight, using the formula 1 oz. + 1/2 oz. per in. of car, they are 5 in. cars and should weigh 3.5 oz. but only weigh 1.3 oz. They do derail occasionally, but that may be a turnout problem. We're gonna get all new turnouts when we set this up permamently. There is a box car in front of them and a caboose behind them, so I don't think it's a heavier car pushing a lighter one. If I can find a new turnout in town today I'll replace it then move on to weighting the tank cars.
Thanx again
Mike