stop), but fault-tolerant aspect design can help the engineer take a safer course of action before the failure of a signal becomes apparent. While not all aspects are fault-tolerant, the green lamp on the topmost head is only used by the least restrictive signal aspect, "Clear," so there is no case where a failure could accidentally display a clear aspect. Where a signal aspect incorporates a flashing lamp, the flashing lamp is always applied to less restrictive signals. This is to prevent a stuck flashing relay from accidentally upgrading the signal. Some signaling logic incorporates "bulb out" (lamp failure) or other fault detection, to attempt to display the most restrictive aspect in case of a fault. However, this feature is not required nor universally adopted. Signal types Semaphore signals Semaphore signals were first developed in England in 1841. Some U.S. railroads began to install them in the early 1860s, and semaphores gradually displaced other types of signals. The Union Switch & Signal company (US&S) introduced an electro-pneumatic design in 1881. This was more reliable than earlier, purely mechanical versions, and more railroads began to use them. At that time, however, they were considerably more expensive than Hall disc, or "banjo", signals. By the end of the 19th century, particularly as trains became longer and faster, and railroad lines grew more congested, the banjo signal was considered to have a single and terminal flaw: visibility. The internal disc was difficult to see in foggy weather and when snow clung to the glass panel. Earlier types of electro-pneumatic semaphores made by US&S had seen some limited application by 1880 as automatic block signals. The need to maintain air pressure in the long pneumatic lines eventually led the railroads to discontinue their widespread use as automatic block signals. However, these types did see long service in interlocking plants. Early semaphores also had limited range with manual wire operation and poor reliability in bad weather. Thus some railroads continued to use disc signals where automatic block signal operation was needed between manual block stations as borne out by period rule books well into the 1920s and beyond. By the early 1890s more railroads began installing electric motor-operated semaphore signals, which were visible at distances of thousands of feet, during the day and under inclement weather conditions. In 1893 the high voltage, electric motor automatic block signal semaphore made its debut. By 1898, the US&S