A train loses speed through four primary mechanisms: Mechanical Braking, Dynamic/Regenerative Braking, Friction, and Aerodynamic Drag. In 2026, most modern trains use Regenerative Braking, where the electric motors are reversed to act as generators; this creates "electromagnetic resistance" that slows the train while sending electricity back into the power grid or onboard batteries. For emergency or final stops, Pneumatic (Air) Brakes apply physical pads to the wheels or discs. On a "passive" level, a train loses speed due to Rolling Resistance—the friction between the steel wheels and the steel rails—and Aerodynamic Drag, which becomes the dominant force at speeds over 160 km/h (100 mph). If a train loses speed unexpectedly without the engineer's input, it is often due to "power sag" from the overhead lines, wheel slip caused by wet leaves or ice on the tracks, or the activation of an automated "Positive Train Control" (PTC) safety system that enforces a speed limit.