Edit post

Edit your reply below.

Post Body
Enter your post below.
First off, let me say that all these questions and my answers revolve around single-main-track operation. Multiple main tracks or double-track (they are not the same thing!) are different. Double-track is directional, handed, one-way. Multiple main-track is either direction, either track. (GCOR has recently fuzzied this distinction, which I don't think is a good idea, but what do I know.) In DTC, a block has fixed, permanent limits, listed in the timetable and posted at trackside. In TWC, a block has temporary limits that last only until the warrant is fulfulled or annulled. In CTC, the block is between absolute, controlled, signals. They must be controlled -- you can have an automatic interlocking in the middle of CTC (and often do), but those are not controlled signals and therefore cannot be the beginning or end of an authority. Scott, your example of following trains would work like this. What you've described is a main train operated by CTC. Both trains can have authority within the same CTC block so long as they are (a) directional and in the same direction (following); (b) directional and moving away from each other; (c) both on track-and-time moving in either direction. The CTC signal acts like an ABS in the example of a following train. Suppose the first train has just passed the absolute signal. It displays red, because the signalling block it protects is occupied. Now the first train passes the first intermediate beyond the absolute. Now the absolute can be cleared for the following train, and will display yellow. The dispatcher may have already requested the signal, but it won't clear until the first train is past the first intermediate. And so forth -- the absolute goes to flashing yellow, then green, as the first train proceeds past additional signals. To just complicate things further, most ABS systems in the U.S. today are APB-ABS -- absolute permissive block. That is, they are Absolute for opposing moves and Permissive for following moves. Early single-track ABS systems were not ABP, and allowed only one train at a time to move between sidings -- not so hot when you want to run a lot of trains. A dispatcher can also talk the following train past the signal, which you would do, for example, for a helper that needs to come up to the first train and couple on. I don't have the rule book in front of me, and after three years I'm increasingly fuzzy on details, but I believe you do not have to tell the first train you are doing this, because the following train will by rule have to move at restricted speed, and if the first train wants to make a reverse move (which it can within the same block without permission) it also has to do it at restricted speed. But I would tell the first train anyway that there was a move permitted behind them. The key point to remember about CTC is that all it really is, is an ABP-ABS system that a dispatcher can make requests of. The dispatcher cannot actually clear a controlled signal, he can only REQUEST that it be cleared. If the signalling system sees no conflicts, it will grant the request. All the dispatcher controls is the direction of movement and the position of controlled switches (normal for the main track, reverse for the siding or spur or junction track). The dispatcher can prevent controlled signals from clearing -- that is, the controlled signal will not clear on its own unless the dispatcher has asked it to. That's essential so you can protect a track-and-time in a block -- it wouldn't do to have a maintainer out there on track-and-time and the signal clear for a train to enter the block! The dispatcher has no control over ABS signals at all. They do their own thing. As for TWC and DTC, trains can only be within the same limits if they are joint, both operating at restricted speed.
Tags (Optional)
Tags are keywords that get attached to your post. They are used to categorize your submission and make it easier to search for. To add tags to your post type a tag into the box below and click the "Add Tag" button.
Add Tag