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[quote user="Doublestack"]In looking at employee timetables, for a given section of the railroad, I've found where the timetable will show sections as single track, sections as double track (DT) and other sections as two main tracks (2MT).    What would the difference be between double track and 2 main tracks? I'm guessing that one might be restricted to directional / current of traffic running vs. the other may be signalled in both directions, but that's just taking a guess. Any help would be appreciated. Thx,Stack[/quote]A more important set of differences are capacity and cost.Capacity:  2MT CTC is typically good for 70-75 trains per day average capacity, whereas double track is typically good for 45-50 trains per day average capacity.  The difference lies principally in track maintenance restrictions.  When engineering has track & time with 2MT CTC the railroad only loses to revenue service the section of main track between the nearest control points, which is typically 10 miles or less in U.S. practice, and trains running on the single track that remains in service can continue to run on signal indication at maximum authorized track speed, with following moves also proceeding on signal indication at maximum authorized track speed.  With double track the distance between control points is usually much longer -- often as much as 100 miles -- and while there might be hand-throw crossovers or center sidings with a spring switch at one end subdividing that distance into 10-20 mile segments, the crossovers and center sidings are typically hand-throw, if signaled typically nothing more favorable than a lunar, and thus restricted to 20 mph or less.  One-half of the trains will be running against the current-of-traffic and cannot proceed on signal indication and require a verbal authority, and following moves against the current-of-traffic are greatly limited in frequency because they have no signal protection against preceeding moves (no flag protection).  Trains running against the current of traffic are restricted to 49 mph (freight).  (Theoretically when single-tracking occurred the railroad could run as manual block and the 49-mph limit would no longer apply, but that introduces some new limitations on capacity.  I don't know of anyone that has ever tried this.) The 70 trains per day average for 2MT CTC is based on 60% of theoretical capacity.  Some people in network planning and traffic modeling, particularly some of the people selling their expertise in those areas, tout a capacity of 75% of theoretical is attainable, presumably if you adopt their operating methods.  In our long experience 60% has proven the sustainable upper limit.  Above that level the network becomes fragile.  While one might briefly sustain a higher number, inevitably weather, mechanical, or other outside factors soon intrude.  Because the network has little freedom to adapt to the unforseen, congestion if not gridlock rapidly ripples outward from the point of injury and trains can no longer be accepted at the network entrances ... and capacity goes to 0%. Cost.   No one has installed very much double-track ABS in about 50 years, but if one was to do so today the expenditure for signaling and control points might be about 50% as much as would be spent for 2MT CTC, assuming that 2MT CTC would include a universal or split crossover with four #20 or #24 movable-point frog turnouts every 10 miles, and double-track ABS would include two #14 or #15 handthrow crossovers every 20 miles, and control points every 100 miles or so (at the crew changes).  In rough terms the difference is about $1 million per mile.  That of course is on top of the $1 million per mile for each track exclusive of turnouts, bridges, drainage structures, earthwork, subballast, grade crossing warning devices, grade separations, ABS, etc. S. Hadid
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