A single wire is laid adjacent to each rail in each separate track section within a railroad interlocking. The wires in each section do not form a complete loop but rather are connected to contacts of the switch circuit controller associated with the track switch at each end of that section, specifically to those contacts which close when that section is included in an established route. When a route is established, the circuit controller contacts connect the wires of selected sections into a completed loop circuit parallel to the rails along that route. This loop circuit is supplied with speed command signals to which train carried speed control apparatus responds in the same manner as to signals transmitted conventionally through the rails.