Following are the topics/questions discussed in this section of the OpSig Primer. You can click on a question to jump directly to that topic. You are invited to address further questions or comments (please refer to question number) to firstname.lastname@example.org.
Question 1: I never quite understood how the yardmaster, working a large classification yard, knows when to send the cars residing on a specific classification track out onto the layout: Is it based on a mere visual reading, when x number of cars accumulate on track 3 the track is then sent on its way? Or is it based on a specific time schedule: At 10am and 4pm track 3 is sent on its way?
Would each class track have its own timetable or would they ALL be sent out at 10am and 4pm?
Answer 1: Let me make a subtle distinction: classification tracks correspond to 'blocks,' which are cars for groups of destinations that are moved together by particular trains. I say trainS because there may be more than one train which takes a particular block if it has much volume. So classification track #3, 'Bostons,' is pulled and added to the 10 am train and the 4 pm train because those are the trains that go to 'Boston.'
However, track #4, 'Mechanicvilles,' gets pulled only at noon because there is only one train a day to Mechanicville.
On other than small layouts there is usually a schedule of trains that run regularly specifying what blocks of traffic they handle. The trains might be numbered in the timetable or might be dispatched as 'extras,' but they are planned and expected. The plan is constructed so that all the regular flow of traffic for the various destinations gets handled in a predictable manner. Once in a while the yard may find it has a big bulge of some particular group of cars beyond what the regular trains can handle. Then (in consultation with the dispatcher and if there is a spare locomotive) a true (unplanned) extra may be called to move the overflow, but this would be less often than once a 'day.'
Class yards have been compared to the postoffice sorting rack. There is a pigeonhole (track) for each mailman route (block), and incoming mail (cars) are sorted as promptly as possible into the proper pigeonhole so they are ready when the carrier leaves on his route. In the old days busy routes had morning and afternoon carriers so the pigeonhole was pulled twice daily. Once in a while (Xmas) there is a big surge of mail and extra carriers (trains) might be employed.
So the YM isn't working in a vacuum but is carrying out his part of a prearranged transportation plan.