I'm not an "expert" on reducing friction losses, but I think you need to consider viscous friction (due to contact of the vehicle with air, i.e., air drag) and dry friction (due to contact between the vehicle and the track, and the friction of the car axles).
As far as air drag is concerned, you could probably benefit from the designs of aircraft and motor vehicles. Although the scale is different, designers probably now what those factors are when they use wind tunnels to design the contours of the vehicle.
As far as dry friction is concerned, you need to look at the wheels of the vehicle and the type of surface of the track. Since gravity is the force propelling the vehicle (as opposed to wheel torque and static friction) I would think you would look for track surfaces that have the lowest coefficients of friction.
Then there are the materials and design of the wheel surfaces and the "axles". I remember working with my son on cub scout car racing and the benefit of using graphite on the wheel axles to reduce axle friction.
These are only some observations. They are not obviously mathematically rigorous, but perhaps they can help guide the analysis.
Regarding the use of magnetic levitation, as has been suggested, correct me if I'm wrong, but aren't you looking for ways to minimize lap time without the presence of forces other than that of gravity and friction?
I think air drag is a minor error in such experiments
You're probably right. You might consider a simple experiment to prove it. Configure a vehicle to try and deliberately introduce air drag without influencing other factors. Perhaps install a lightweight "sail" on a vehicle and see if it measurably slows it down.
One last major consideration. Are there any constraints on the design of the vehicles on the track. I mean, if there weren't any you could just use stainless steel balls as @user9976437 suggested. If this is a competition, then there must be some rules. And that goes for magnetic levitation as well.
So I guess the first question any of us should have asked you before answering your question is what are the rules of the competition, if any.
Hope this helps.