Railroad signals became an essential part of a complex transportation network that allowed multiple trains to operate safely and efficiently on shared tracks, and some basic design elements developed nearly two hundred years ago remained in use. Beginning in 1915, the Pennsylvania Railroad used position lights with six lamps in a circle around a central lamp to control train movements, simulating an earlier semaphore system that indicated stop, clear, and proceed with caution. Their indications were determined by weak electrical currents running through the steel rails that changed depending on whether another train was on them. Signal lines on either side of the track transmitted signals for railroad companies and allowed tower operators to activate signals remotely, while railroads also leased extra space on the poles to telegraph and later telephone companies. During the twentieth century, remotely activating switching equipment gradually replaced human operators. Whistle posts, among the earliest forms of railroad signaling and still in use, were traditionally placed about one-quarter mile before a public crossing, where they instructed engineers to sound two long, one short, and another long whistle so people would clear the road ahead.