Track brakes, similar to those used on surface tramcars, may be installed on heavy downgrades underground to supplement other braking systems; they apply blocks to the rails by mechanical, pneumatic, or electromagnetic power. The normal shoe brake must be designed to work in conjunction with the track brakes so that the wheels are not skidded when the track brakes utilize part of the weight of the locomotive. Electromagnetic track brakes may utilize the braking currents produced in rheostatic braking to excite the electromagnets, which are then pulled down onto the rails and produce a strong retarding pull.