Usually that is the case. Especially if you shop around now for new items. I bought some used K line. They are ok but not great. I have a wider tolerance for rolling stock than engines. This is true for all scales. I normally don't shop around for other manufacturers. I try to go with collector/project list and try to stay on course.
I have had trouble with Model Power rolling stock and engines. They tend to be of the same quality as entry level (kit) Bachmann or Life Like. Most are to light and therefore derail easily. The plastic axle makes them have a higher rolling resistance. The couplers that are included are lower quality. The printing tends to be less sharp. Just my two sense. If you want to build up a collection of good quality cars for little cash find a train show in your area. A used Athern blue box is better then 99% of the Model Power cars.
Another way to go is to build the cars from kits. I have found that Roundhouse kits are of good quality. They also as sold for $5 - $8 dollars a piece at my local hobby shop. If you can find them Athern Blue Box kits are of excellent quality for the price. Walthers kits tend to run a little more but they equal a higher level of car than the others.
Staying with brands like Athern, Atlas, Rounhouse will help to keep things trouble free.