A Brake Lever is a handle which is used to apply the brakes of a locomotive or an entire train.
There are several different types of brake levers used for different purposes.
Train Brake - The main brake which controls the train's vacuum or air brakes. On some locomotives this also triggers the locomotive brake.
Locomotive brake - The locomotives own individual brakes which only stop the locomotive in which the brakes are applied. The locomotive's brakes are usually a combination of normal mechanical friction based brakes and dynamic (or rheostatic) brakes which maked use of electric motor's ability to turn into a generator at any time, the energy generated is then either discharged into the overhead wires to power other electric locomotives or, in the case of diesel locomotives discharges into a resistor grid.
Emergency brake - A type of brake which is usually just a notch on the train brake lever that more rapidly haults a locomotive or an entire train.