Steel wheels on steel rails provide sufficient traction (grip) in normal circumstances. When the torque applied to driving axles exceeds the traction capacity, the wheels lose grip and begin to spin. Wheel spin is more prevalent where the tracks are slippery (snow, dust, wet leaves, etc) or the driving axles are not universally connected and rotate uniformly (diesel hydraulic locomotives).
Sand may be used to increase the traction between wheel and rail and some locomotives have detection systems to deal with wheel spin.