How can trains travel in such a frictionless metal tracks?
Weight of the locomotive creates a burden on the wheel, and through it on a rail. Therefore, the motion between wheel and rail friction force (traction), and the wheel does not slide, and rolling on the rail. The heavier the locomotive and the stronger its wheels are pressed against the rail, the larger train it can pull. Of course, the locomotive engines should be at the same time powerful enough. But if the engine would be too light, it would not be able to lead a heavy train, no matter how powerful engines it has. The wheels of the locomotive will not be strong enough to cuddle up the rails and start to slide, "slip."