Railroad signal engineering has long focused on controlling train movements, preventing collisions, and ensuring safe operations. The Camden and Amboy Railroad advanced railroad safety when it introduced the first mechanically activated control system, or interlocking, in Trenton in 1870. After the Pennsylvania Railroad gained control of the Camden and Amboy Railroad in 1871, it originally used semaphore signals, whose movable arm indicated stop, caution, or proceed by horizontal, diagonal, or vertical position. Following the introduction of the electric track circuit in 1872, railroads began installing electrically activated signals across the country. After 1910, some railroads adopted color-light signals using red for stop, yellow for caution, and green for proceed, but because early color lights were not always easy to read, the Pennsylvania Railroad developed and installed the position-light signal system in 1915, using horizontal, diagonal, and vertical rows of three lights to represent stop, caution, and proceed. Train movements over the Rancocas Creek swing span bridge were controlled by four position-lights, with one distant light signal at each approach and one home, or near, light signal adjacent to each end of the bridge to show whether the swing span was open or closed and to protect trains crossing the creek. These position-light signals and other antiquated signals were later removed and replaced with state-of-the-art signaling systems during construction of the light-rail line, while the two distant signals were donated by NJ Transit to the Cape May Seashore Lines railroad, where they continue their historic function of controlling train movement.