Today, the primary motive power unit employed by U.S. railroads is the diesel engine. The steam locomotive has been phased out, and the use of electric traction has declined markedly. Until recently, the trend was against the further extension of electrified operation because of high initial costs. Further, the diesel locomotive can be operated throughout a railway system??in both electrified and nonelectrified zones. Now, however, with the advent of extensive electric power grids, rectifier locomotives, and ambitious plans for extensive rapid-transit systems, electrified operation has obtained a new lease on life. Signal system development has brought notable improvement in railroad safety and operations. Most important in this category is centralized train control, by means of which a single tower can control all switches and signals over more than a thousand kilometers of mainline track. And with automatic train stop equipment and speed control, a train running through a stop signal or exceeding a speed limit can be electronically stopped or slowed, thereby eliminating the possibility of human failure. Two-way radio communications have done much to expedite freight-train operations. Many locomotives, cabooses, and signal towers are equipped to permit train conductors to talk to engineers or tower personnel, and to receive orders while en route.