Five ounces = 141.75 grams. That seems excessive if you're adding that weight to each of your 15 wagons as you're expecting the loco to haul the equivalent of over two bags of sugar (albeit with less rolling resistence!).
To put it in perspective, P4 finescale modellers are advised to have 25g (0.88 ounces) per axle in order to prevent derailments, and that would be considered overkill for standard OO gauge (our Accurascale 24.5t hoppers weigh 26g in total and we have had no reports of derailment issues from any customers, including finescale modellers). If you're experiencing derailments, it's more likely due to an issue with a particular wagon (a wheelset may have the incorrect back-to-back measurements, for example), or there may be a problem with the track - either a manufacturing flaw or in the way it was laid.