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[quote user="Datafever"]Thank you for the clarification, Ed. Another question (I hope that this wasn't answered already and my eyes just glossed over it) - do both DT and 2MT have sidings in the same way that a single main line would have? [/quote]It wasn't asked or answered already, so you didn't miss it.On a 1MT line the primary purpose of a sidings is to meet opposing trains.  Siding spacing and length absolutely, positively, determines the capacity of the single track line.  The running time between sidings that will fit the trains you are trying to meet is the ultimate limit on the capacity of the line, period. You can use a siding in 1MT territory for overtakes, but only if there's a lot of spare capacity. On a double-track or 2MT line, train meets are handled by the other track.  Because double-track lines had very limited ability to use the other track for wrong-way moves, sidings were necessary if one was to have any overtakes outside of terminals, at least without really making a mess out of traffic on the opposing track.  Back in the era when these double-track lines were engineered there were a lot of passenger trains on these lines, and the sidings were critically important so that freights could run against the schedule of a 1st Class train and not delay it or themselves.2MT territory has complete flexibility of which track is used for which direction, so the imperative for sidings needed in double-track territory can be greatly diminished, at least if traffic is not so high that running a train "wrong main" to do an overtake can be done without inflicting serious damage on the schedule adherence of trains coming the other way.  If, for example, the crossovers are 10 miles apart, the overtaking train is going to use up the other main track for at least 20 minutes during which any opposing traffic that wants to enter that 10 mile stretch is going to stop, while following trains bunch up behind it.  The overtake if not done right can easily be a net loss, because it bunches fast trains behind slow on the other main, which creates needs for more overtakes, which in turn creates more need for overtakes on the track you were trying to unbunch in the first place! Sidings in 2MT territory are also used for, block swaps, storage, terminal hold-outs, and the like, and are useful when one track is given away to maintenance-of-way because trains can meet there.  (Same applied to double-track, too).  If it's a controlled siding, all the better.  Some ABS double-track roads seemed to prefer outside sidings, in the West notably the Santa Fe, and some seemed to prefer center sidings, in the West notably the UP Overland Route.  Generally the leaving switch (remember this is current of traffic, directional running) was a spring switch.Single-track CTC railroads that are converted to 2MT, such as large portions of the BNSF Transcon and UP Sunset Route during the last few years, retain some of the old sidings -- if they're on the opposite side of the existing main track from where the new main track goes; otherwise they're usually retired.  They're nice-to-haves from an operating department perspective, and provide another place to get the track inspector or track machines out of the way so trains can run. But you don't absolutely have to have sidings in double-track or 2MT territory.  If you think about it, one of the main tracks between crossovers in 2MT territory is in effect a "siding." There's a lot of discussion in the business about the value, use, location, and need for sidings in 2MT territory.  And if they're to be retained, there's discussion about how the turnouts should be arranged.  BNSF has built some lately with nested turnouts, which in effect gives one three main tracks for the length of the siding.  But that's expensive and there's debate over whether the flexibility gained is worth the money spent.  Building a 10,000' outside siding in 2MT territory, from scratch, is a $2-3 million proposition.  And it's difficult to demonstrate in advance how that expense will repay itself.  One philosophy is that if you really need that much capacity, you might as well build a third main track.  It's not an easy thing to figure out. S. Hadid
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