"How is wheelslip controlled in a steam loco?"
Answer :
By the driver shutting the throttle/regulator when he hears the wheels
start to spin - It is purely a question of driver control. On a
Diesel-Electric, the current draw of individual or groups of traction
motors are compared - the motor (or group) which draws proportionally
less Amps than the others is deemed to be in a state of slip - and the
power is reduced (Some systems - EMD Super Series for one - measure
known wheel speed against ground speed as registered on a Doppler
Radar). In the case of both Steam and Diesel-electric or diesel
hydraulic locomotives, sand is applied to the wheel/rail contact point
to increase friction - this is either controlled automatically, or by
the driver.
Hope this helps !!