How the train moves on track without slip?

Well, trains don't fly off the tracks because of the wheels, the part that rides on the track is notched out, leaving a back-plate that holds it on the tracks.

When accelerating, the wheels can very easily slip. Even with expert engineers, the wheels will slip sometimes. Since it's metal on metal, there's a low amount of friction. The train moves because of the extreme weight, forcing the wheels against the track. Often times when starting from a stop, the train will spin it's wheels a bit, but once it gets up to speed, momentum makes things a lot easier; at least when accelerating. Decelerating, or slowing down/stopping is very difficult with a train, due to that same lack of friction. A train with many heavy cars can take a mile or more to come to a full stop.

The reason that trains use such a low-friction medium for their wheels is because of torque. The amount of torque when getting the train up and going would be immense, and exponentially more when stopping. It would likely rip the wheels and axles right off of the train.

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With respect, not quite. The wheels are not "notched" but flanged (the "back-plate", integral with the wheel's steel tyre, not a separate component). In fact the flanges are a sort of back up because the wheel rim and rail profiles are so designed that each wheel-set (pair of wheels and the axle on which both are rigidly mounted) is self-centering on the track. The flanges play a relatively minor part except on very sharp, slow curves; they don't stay in contact with the rails.

The effort needed to accelerate or decelerate anything is to overcome inertia, and the reason a train is so efficient is its low rolling-resistance.

Torque is turning-moment. The driving torque on a train is applied via gears to the driving-axle, and there is no way it could rip the wheels off unless the components are actually flawed in some way because the whole machine is designed to withstand more than the maximum effort available from the engine or motor. Even if the wheel slipped on the axle, the axle would stay in place, in its bearings.