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Daniel: A good question. I'm not sure I can explain it satisfactorily (can someone else?). At this point, you really need a rule book to go into this kind of detail. I have to partially start over to try to answer it. An ABS -- automatic block signal system -- does not convey authority to occupy the main track. It merely advises of track conditions ahead. The following things will turn an ABS signal red: a broken rail (most of the time), an open switch, and a train in the block. This gives you a higher degree of safety than no signal protection, and the FRA (and before that the ICC) allow you a higher maximum track speed as a result. Since an ABS system conveys no authority, you must have something that gives it: a train order, track warrant, DTC authority, yard limits, timetable, signal indication from an absolute controlled signal -- one of these. These signals are called automatic because they respond to track conditions entirely on their own. They are not controlled by anyone. A manual block is merely a paper system that divides the railroad into blocks. It's called manual because a human assigns the block. The block limits are established and fixed, but they only exist on paper (and a signpost at their entrance). Unlike an ABS, they do not automatically clear themselves or assign themselves. And unlike an ABS, they DO convey authority but DO NOT tell you anything about track conditions. In manual block, by rule you cannot have two trains in the same block at the same time unless both are at restricted speed (prepared to stop in half the limit of vision short of train, engine, red signal, men and equipment, and open switch, keeping a sharp eye out for a broken rail and not to exceed 20 mph). In ABS-TWC, there are no blocks per se, only "signaling blocks" which have no authority attached to them at all. TWC rules allow you can have two trains in the same signalling block at the same time without both being at restricted speed, so long as train ahead is directional and moving away from the following train, and so long as the trailing train is running at restricted speed. Thus it is less positive of a system, and so is restricted to 49 mph maximum unless it's got a ABS system on top of it. If you pencil this out, you'll see why manual block is more positive. Since you can't put two trains in the same block without both going to restricted speed, a following train cannot enter a block until the train ahead has completely left it. So there will usually be a full block of space inbetween the trains. But in TWC (with or without ABS), you can put the following train right up to the train ahead's rear end all the time. Take a look again at what Mac pointed out in a previous answer, that CTC sits on top of ABS. In between the absolute signals at control points (no number plates) are ABS signals (with number plates). An absolute signal gives you authority all the way to the next one, regardless of how many ABS signals are inbetween, and regardless of what the ABS signals might indicate. The block, in this case, is absolute signal to absolute signal. I think where you're getting confused is that there are two types of blocks -- one that grants authority, and one that's a wiring block between two signals. The first grants authority, the second doesn't. They can be the same physical dimension, or not. Laps are overlaps of authority. Since an ABS system conveys no authority, they cannot create laps. They might protect a train crew from them, however. If two opposing trains both have authority to occupy the same piece of track -- they have a lapped authority -- the signal system will tell them that there is something out there, first by turning yellow, then red. To pass the ABS signal the train must reduce to restricted speed. In the worst case scenario, assuming everyone is obeying the rule and really IS running at restricted speed, the two trains will meet and the two front couplers will kiss . No damage done, but clearly a rules violation by someone. Ain't no shame in asking questions, my rules instructor used to tell me: "The only dumb question is the one you didn't ask."
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