A little less drag for the propwash on a curve, more clean air to some extent. How the curve wheel drag compares to straight, no idea. Probably a wash as for efencivy.
Have you thought about a loco power type pickup from the tracks. Have to see if the prop would get enough to provide the thrust needed, along with any voltage difference. Would be easier than a battery in the car.
Haven't messed with trains since "N" scale started to get a following back in the late '60's