Actually, the voltage on the track doesn't change the characteristics of a high resistance joint. The higher voltage just has makes the voltage drop less noticeable.
If you have a 1 ohm resistance at a joint, and you're running 6V on the track at a one amp load, you're losing about 16% of your voltage through the joint. However, if you're running 12V on the track at that same one ohm load, you're only losing about 8% of the voltage across the same joint. Obviously, this will have a lesser effect on running.
