Pulling the chain just releases the brake pressure and causes discontinuity in the brake pipes. It wouldn't stop train instantly.
In an normal scenario two things might happen. Either the loco pilot upon seeing the discontinuity in brake, would apply the brake immediately and stops the train or the brake pressure keeps lowering and when the pressure notch in the master controller in the engine goes below threshold, an auto brake is applied bringing the train to an halt. Otherwise the train would take a long time to decrease the speed on its own after pulling the chain.
But in this incident, both option was not possible. Also the train was running on down gradient which would have impacted the chain pull.