that is set, the trip arm remains raised and trips the train to a stop. If the train approaches at a speed equal to or lower than the set speed, the trip arm lowers before the train arrives, and the train is able to proceed without further hindrance. Some timed train stops require the driver to acknowledge a stimulus before the trip arm is lowered on a yellow signal. For sections of track with lower speed limits (15–20 km/h) a simpler construction is also used. The trip arm rotates freely on a horizontal axis with a counterbalance attached to its lower end. If a train's speed is low, the arm will be rotated by the trip cock with a force insufficient to initiate braking. But if its speed is too high, force will be large due to the counterbalance inertia, causing the brakes to be applied. Fixed train stops With fixed train stops, the trip arm cannot be lowered. Fixed stops are positioned close to the end of a dead-end track, to stop a train before it runs out of track. They may also be used at the end of track sections beyond which certain trains should not pass, such as the end of electrified territory (e.g. Hamilton, NSW), or to test the automatic brake and tripgear of trains departing certain locations, e.g. storage sidings, near buffer stops. A fixed train stop that is the last one on a running line in the reverse direction may, despite its name, be suppressed, as is the case with the associated "Fixed Signals" on the Sydney network. Suppression is needed because in Sydney, the rear trip cock on a train is always lowered, while in Melbourne, by contrast, suppression is not required because the trip cock at the rear of a train is always raised clear of any wayside trip arm. Some railways and rail transit agencies use fixed train stops to protect workers in work zones by temporarily applying them at either end of the zone, preventing trains from incorrectly entering the work zone. Installations In 1901, Union Switch and Signal Company developed the first automatic train stop system for the Boston Elevated Railway. This system was soon adopted by the New York City Subway and other transit systems in the United States. Similar systems were installed around this time on the London Underground system. Because of its mechanical nature, the train