Photoelectric Detector (PED):
1. These detectors are used to sense the position of trains on the track.
2. A "look back" and "look ahead" technique is used.
The detector at the end of a section of track will be behind a train.
3. The detector at the start of a section of track will be in front of a train.
4. When a train crosses the light beam at the end of a section of track, the "look back" detector will inform the Transformer and Switch (TSW) combination and it will turn off the power to that section of track.
5. When the train reached the "look ahead" detector coming up in front of it, that detector will inform the Transformer and Switch (TSW) combination that belong to that section of track, to place power on it.
6. The result is that only suitably short sections of track will carry electric power at any given time.
7. The safety margin gained will out weigh the extra cost and still be less than that of single function overhead lines.
8. Infrared cameras on the trains or the track can serve to inform train drivers that there is something moving on the track directly ahead and cause a driver to sound the horn according to the trains breaking distance.