Dark territory

Dark territory is a term used in the North American railroad industry to describe a section of running track not controlled by signals. Train movements in dark territory were previously handled by timetable and train order operation, but since the widespread adoption of two way radio communications these have been replaced by track warrants and direct traffic control, with train dispatchers managing train movements directly. Today most dark territory consists of lightly used secondary branch lines and industrial tracks with speeds ranging between 25 miles per hour (40 km/h) and 40 miles per hour (64 km/h); however, there do exist a small minority of main lines that fall into the category. The term can also apply to automatic signaled lines where trains are running against the normal flow of traffic.

In the UK and Australia the term applied to rail track where the signalling system does not pass the signal indications nor track occupancy back to a signal box. As such the position of trains is not visible to signallers, and so the track is "dark".

The primary safety concerns with dark territory stems from the lack of any form of direct or indirect train detection along the route. Train detection systems such as track circuits not only alert other trains to the presence of a potential hazard, but can also alert dispatchers or other monitoring systems to the same. Dark territory also lacks the ability to control or lock switches onto the main track, detect misaligned switches, broken rails or runaway rail cars. In most cases these drawbacks are mitigated by the light traffic and low speed of the trains in dark territory, but a runaway train (such as the crude oil unit train in 2013's Lac-Mégantic derailment) will not respect limits on speed and is not detectable by rail traffic controllers on a line with no signals or track circuits.

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Wikipedia