Each engine is provided with a whistle (fig. 14) blown by steam
turned on from the boiler, which is used as a signal at any
particular time, especially in tunnels or when there is a fog; there
is also an arrangement by which each engine presses on a lever at
the side of the tram as it passes, and causes a bell to ring at the
station, to announce its approach, when about a quarter of a mile
off. In some cases, as in foggy weather, when the usual signals
cannot be seen, a packet of fulminating powder is placed on the rail,
and this being exploded by the wheel of the engine as it passes over
it, gives notice of its approach, &c. There are other signals, but
these are the chief.