The coefficient of adhesion is
DC locomotives typically require more weight on the driving wheels to achieve the required tractive effort due to the light weight of their motors.
AC locomotives, on the other hand, can achieve the required tractive effort with less weight on the driving wheels due to the higher weight of their electric equipment.
The required level of adhesion depends on the type of vehicle and its acceleration rate.
Rapid transit cars with every axle motored typically require about 5% adhesion for each 1 mph/s of acceleration rate.
DC locomotives can typically attain 28-30% adhesion, while AC locomotives can achieve 33-35% adhesion on a clean, straight rail.