content: Because they're in the same inertial reference frames. It's why everything's settled and not getting flung around when traveling 100mph in a car, or 300mph in a train. Newton's first law states that any object in an inertial reference frame maintains its movement speed unless affected by an outside force.  In the example of a bullet train, the train itself is moving at ~300mph, with friction (from the wheels) and air resistance slowing it down, but the engine imparts an opposite force, keeping the train moving at the same speed. Inside the train, however, the only force imparted on the passengers is the friction from the floor and seats when the train accelerates/decelerates, so they continue moving at the same speed as the train.