I presume you are operating these locos in conventional mode, using the DCS remote, and through the variable channels of your TIU.
Disclaimer: It's been a very long time... My TIU is one of the very first models, and I never upgraded the operating system beyond 3.xx. But if I recall correctly, when using the variable channels, the indicated voltage on the remote's LCD display wasn't accurate.
All bricks are not created equal. Which one(s) are you using? Do you have a traditional transformer to compare against?
The most important test: if you put a volt meter on the rails when the train is running, what does it read? How does that compare to what the remote is telling you? Please try this test and post back with your results.
It's very possible that your locos are drawing excess voltage due to wear, or lack of maintenance. But they could also be operating normally. We can't tell unless we know the actual voltage on the rails. Measuring it during operation with a meter is the best way to find out.