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Daniel: Mac and Dave have done an outstanding job of providing concise yet accurate answers for you, not at all easy given the great complexity of the questions asked you asked. I would highly recommend you search in Google for GCOR (General Code of Operating Rules). You can find it as a pdf, but expect to print out a few hundred pages. It has a list of definitions at the end which will answer some of your questions and explain stuff that we can't get into here, unless we want to retype the whole thing! I dispatched extensively in DTC territory at Kansas City Southern, and learned TWC at BNSF, so I think I can add a few things to Dave and Mac. SP was a major user of DTC, but as far as I know UP has by now converted all SP DTC territory to TWC. KCS continues to use DTC. Of interest, KCS DTC is actually a manual block authority, which allows trains faster maximum speeds. Normally, "dark" (unsignalled) territory is limited to 49 mph for freight, but if you have manual blocks you can exceed that. Originally, all railroads dispatched by timetable and train order. The timetable established authority for every scheduled train, designating meeting points and the exact time a train left a station -- the train could run behind its timetable time, but NEVER early. The timetable stated a hierarchy of precedence: eastward trains (usually) had rights over westward, first class over second, second over third. If all goes perfectly, every train can run entirely by the timetable and a watch, needing only a clearance card from the dispatcher to leave its originating station. You can even run an "extra" train -- a train which has no timetable authority whatsoever, and not even need a train order to grant you authority, assuming the railroad is not too busy. All the engineer and conductor have to do is run against the authority of the scheduled trains, that is, inbetween their times, ducking out of the way into a siding when a scheduled train becomes due. But since things never go perfectly, the train order is needed. The train order modifies the timetable: in effect, it temporarily suspends it for the trains to which it is issued. For instance, if your scheduled train No. 6 is running four hours late, you wouldn't want inferior trains to get out of the way for it four hours early. So you issue a train order stating No. 6 runs four hours late, which is MANDATORY on No. 4 and ADVISORY on inferior trains. That allows inferior trains to modify the timetable this one time and keep moving, and tells No 6 that it cannot leave any point one minute less than four hours behind its timetable schedule. This is necessary because No. 6 might make up some time. Timetable and train orders worked very well back when passenger trains were prevalent. Since passenger trains do the same thing every day, and by nature are not supposed to ever be late, you could run a railroad very efficiently with this method. Most days, you didn't have to issue very many train orders. But once passenger trains disappeared, the advantage of the timetable method of operation evaporated. Now you wanted to run every train as an "extra," that is, with no fixed schedule at all. That meant the work load on train orders got out of hand. So the Track Warrant and DTC authority were invented. All they are, in effect, are simplified train orders, with all the timetable-modification stuff stripped out. Two similar types of oral authority arose to replace the train order and timetable: DTC and TWC. The principal physical difference is that DTC has fixed, constant blocks designated by signs to the right-hand side of the main track at the entrance to each block. TWC has "floating" limits designated by the dispatcher, which can extend between mileposts, station signs, siding switches, or any combination thereof. Basically, any point that's in the timetable can be an endpoint of a TWC authority. You can see how convenient this is: TWC blocks can be one mile long or 100 miles long, just like that. Typically DTC blocks extend from siding to siding, say, from east siding switch to east siding switch. Sometimes there will be additional blocks on the main track between siding switches. DTC is obviously less flexible but less subject to misunderstanding as to where authority begins and ends. They present fewer chances of "missed repeats," where a crew either writes or hears the information from the dispatcher incorrectly, and when they repeat the authority the dispatcher doesn't catch the change. Missed repeats are a common way to create a "lap" (two trains with overlapping authority), and have led to disastrous, fatal collisions, even in recent years. In dispatcher class, we all watched video of a headon collision of two BN trains at Ledger, Montana, a decade ago that killed the headend crewmen, which was caused by a missed repeat. I later read the NTSB report on the wreck. This was a line with one train a day each way. Normally the northward train arrived at its home terminal before the southward train left -- they met in the yard. This day the southward train was early. The afternoon trick dispatcher knew the southward train would leave early, so issued an authority to the northward train to a siding south of the yard. The crew on the northward train was so used to getting the track warrant all the way to the yard, that they filled it out that way and repeated it to the dispatcher that way. The dispatcher didn't catch the change! She OK'd it and that was that. Two hours later, she goes home, and the midnight trick dispatcher comes in, takes the transfer, and when the southward train is ready to leave, he looks at the track warrant book and sees the northward train is cut off at the sidng. So he goes ahead and issues a TWC authority to the southward to the siding. And off the southward train goes. Meanwhile, the northward train, holding what it thinks is a valid authority, runs past the meeting point and in a curve the two meet at a combined speed of 90 mph. The headend crews are killed, and eight locomotives and 40 or so cars destroyed in the collision and fire. The video showed the next day, using side-boom Cats to pick up the equipment and tracked front-end loaders gingerly scooping up spilled grain in the search for the dead crewmen. They intended that video to scare us, and it did. All you have to be is inattentive for one second, and you kill someone. I'll disagree with Mac on the "easier" part of DTC; they are actually just as time consuming to issue as a Track Warrant. If you issue more than 50 of these in eight hours, you are busy. I think the most I ever issued in one shift was 80. I never left the console even to go the restroom and never ate my lunch. Authorities to occupy the main track also include yard limits, block register (usually seen on one-train-a-day branch lines), track permits, track-and-time (CTC), and work-and-time (track permit territory). Oh, and by the way, this all just applies on GCOR railroads --railroads that subscribe to the General Code of Operating Rules, which is most of those in the West. Eastern railroads use different rules which are similar but not identical to GCOR, and Canadian roads have yet a third set. I assume Mexico is different again. The difference between an Absolute Signal and a Block Signal is, well, absolute. ANY signal with a number plate is a block signal. It does not convey authority: it only advises of track conditions ahead. Any signal without a number plate is an Absolute Signal. A block signal with a red aspect indicates stop, but then you can proceed at restricted speed. An absolute signal with a red indication may NOT be passed, unless verbally authorized by the train dispatcher. This is a fail-safe method: you can see that a block signal can "become" an absolute signal if the number plate falls off one day, but an absolute signal cannot become a block signal. The only other common type of signal is a Distant, indicated with a "D" plate. These are used to provide advance information as one approaches signalled territory or an interlocking, and do not convey authority or information on the track condition beyond them. They only tell you about the indication of the next signal. One common modification to a block signal is the addition of a "G" plate -- a grade signal. Signals so modified can be passed at restricted speed WITHOUT first stopping, and are used on heavy ascending grades to avoid difficulties in restarting a train. Some railroads, like the D&RGW, used to use a lunar head for the same purpose (you could see them farther away), but I think that practice is going away. Your last question addresses how railroads keep track of locomotives and cars. Locomotives are tightly tracked, because you WANT to keep track of them. They're expensive and critical. Large railroads have a "power desk," which knows where every locomotive on the system is at: on a train, in a terminal, in a shop. Each dispatcher's console includes a trainsheet (it's displayed electronically now) which lists every train they are currently handling. Information for eacn train includes the locomotives. The train electronically "OS's" (reports its location) to the train sheet each time it passes a control point in CTC territory or gets a new TWC or DTC authority, and the screen updates itself. So you can look at the trainsheet and know exactly where a given locomotive is, or at least where it was four or five minutes ago, which is good enough because you know which direction it's going. That information is accessible to the power desk, if not directly updating into their display screens. So at a glance they get the whole picture. Cars are less tightly tracked, because they don't need to be. All you need to know is that it's going in the right direction and on the right train, and when it left the last terminal. Each train has a trainlist that shows every car in that train -- hopefully the list is accurate and not missing some cars that are there, or showing cars there that actually aren't, and in order. Railroads used to employ clerks to check car numbers as trains entered and left yards, by standing next to the track and writing them down. Now they use electronic readers which check the transponder attached to each car, as well as fixed video cameras which allow you to look at the train and write down the car numbers from the office. Very frequently, you'll run a train list as a train passes a reader, because you want to verify its consist. Beyond that, car tracking gets hard to explain. There's a broad variety of methods, ranging from the yellow legal pad on your desk to a computer database with automated inputs, with inputs ranging from the reader at the end of a yard to GPS transceivers that constantly update their location. It's a fair question to ask why railroads don't have the location of every car at their fingertips every second, and the answer is that they don't need that information -- why spend money on something unimportant? For example, if you have a 117-car unit-train set cycling between the same mine and the same power plant 150 times a year, all you need to know is that no cars have entered or left the set (the legal pad works fine for this) and a rough location of the train. It's usually irrelevant what order the cars happen to be in. In this instance, railroads often use the "tracer car" system: they track just one car in that train and assuming the other 116 are still coupled to it.
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