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To add to Chad's description: *ALL* "methods of operation are defined by a block, that is, a section of track with fixed limits. The *only* difference between CTC (and similarly interlockings) and the other common methods of operation (TWC, DTC, manual block, etc.) is whether the authority to occupy a block is granted by a signal indication or by an oral or written authority. The *only* significant difference between TWC and DTC is that the block in TWC has temporary limits chosen and designated by the dispatcher whereas the block in DTC has permanent limits. There are "floating block" methods extant in laboratory settings (the train carries the block with it) that may eventually replace the fixed block methods, if the technology can be implemented at a reasonable cost. There is nothing intrinsically unsafe about operation in dark territory: a train crew can exceed its authority in CTC limits just as easily as it can in dark territory, in the former case by disregarding a signal indication and in the latter by disregarding the oral or written instruction on the TWC or DTC form. In my experience I much prefer DTC as the block limits are always the same, and as the train crew and dispatcher have to keep their head in the game much more than in CTC. Better yet was timetable and train-order operation. The flexibility of CTC has devolved to a crutch allowing undisciplined operation and ad hoc scheduling at every level of the railway operation, especially at the top where this technology facilitates impulsive operating policies that are born fresh every morning in reaction to the fad du jour or the squeakiest customer wheel. Dark territory exceeds signaled territory in total mileage, though most dark territory is light-density branch line and tertiary main-line trackage. The major advantage of CTC is it offers a significant capacity increase over TWC or DTC. Chad, a couple of notes: 1. ABS doesn't necessarily prevent collisions -- it only advises of track conditions ahead. 2. It's not accurate to say TWC is preferred. It happens to be more common in the Western U.S. but not in the Eastern U.S. The flexibility of TWC is largely theoretical, in my opinion, and at the expense of significant safety, also in my opinion. S. Hadid
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