Referring to the extension of the ASL Fitzgerald Subdivision siding at Hatley on CSX to 11,239 feet, a question was posted in The Railroad List (RAILROAD@CUNYVM.CUNY.EDU / bit.listserv.railroad), asking when a section of 2 tracks is a siding and a main track and when it is two main tracks.
Generally, the designation siding or main track is a matter of function. The allowed speed and presence of signaling follows the function rather than the other way around. Put simply, a track for normal train movement it is a main track. A track to be used by trains clearing the main track for trains in the same or opposite direction is a siding.
Determining if a section of line will be used as a main track and a siding or as two main tracks can involve the frequency of meets occurring at that location. Unnecessary operation of trains through the diverging side of a turnout is avoided. The wear on the turnout caused by a diverging train is greater than by one on the tangent side of the turnout. Secondly, the speed must be reduced for the diverging movement. Braking then accelerating, perhaps twice, to enter and leave the second track is an expense that is unnecessary if no train was met or passed. Maintaining the track to the same standard as the main track may be an unnecessary expense for an infrequently used track. If the necessary use of one of the two tracks is infrequent, it will probably be a siding.
Sometimes the track designation will also be associated with a choice of the set of rules associated with movement on the track. The procedure for protecting cars left on a main track is much more cumbersome than for cars left on a siding, which is more cumbersome than for cars left on a track that is neither. The same thing applies to switching and to track maintenance. Requirements for the use of a siding in both directions can be much less involved than the use of a main track in both directions. The simplicity of the procedures is generally accompanied by lower speed. That is not necessarily the case though. Curvature and other considerations may restrict track speed regardless of the degree to which the track is maintained or the procedures for operating on it. Usually, the simplicity and lower speed is accompanied by reduced control and greater possibility of delay of trains because of an alternative use of the track. Each of these factors is considered in determining whether a section of a line is single track with a siding, single track and a yard track (or other designation) or two main tracks. Similar considerations exist for additional tracks where there are two or more main tracks.
Vincent (Between Mojave and Los Angeles) on SP had about 8000 feet of double track. There were once 2 shorter sidings (Eastward/Westward). The extended track alloweed meets between longer trains. Vincent straddled the summit of a hill with 1+ percent grades leading to it in both directions. It was not a convenient place for trains in either direction to stop and throw switches. If the second track at Vincent were called a siding, all trains would have used the main track unless required to use the siding, encountering the problems associated with stopping on the grade. A spring switch was used at both ends to keep trains to the right and both tracks had ABS for right-hand running. This arrangement and designation of both as main tracks allowed opposing trains meeting at Vincent to pull most of the train over the summit before stopping. The inconvenient feature of this arrangement was that it effectively prevented the use of Vincent to run a fast train around a slower one because one of them would have been on a main track moving against the current of traffic, requiring the associated protection.
Indiana Harbor Belt had a siding (more recently known as Track 21 secondary track) extending 10 miles between McCook and Norpaul. The track had no signals, hence a lower speed limit than did the 40 mph main tracks. With a 20 mph curve on the main tracks near one end of the siding and a hand-throw switch from the siding into the CNW Proviso yard near the other, the lower speed did not make much difference. The siding (East Passing Track), might be used for a number of trains moving westward to or eastward from the CNW, keeping them out of the flow of trains on the two main tracks. Later in the day it might be used for re-positioning a train from the flow because a yard needed the trains in a different order. Still later it might be used for holding a train that was unable to continue due to being held out of a yard or off of a connecting line. The designation of the track as a siding allowed the Train Dispatcher control over the traffic that would not usually be possible on a track designated yard track, ensuring that traffic was kept moving as planned. The designation of the track as a siding rather than a main track allowed the Train Dispatcher's control of opposing trains to be executed verbally and by message rather than by the more involved process of train orders.
Generally on a single track line using some type of written train control instructions such as Track Warrant Control or Direct Train Control, sidings need be no longer than they trains they accommodate. Those sidings will often be between 4000 and 7000 feet long. On such lines, trains must stop and line a switch to enter the siding. They must often also line the switch out of the siding then stop to line it back for the main track. Not much is to be gained in using switches over which trains can move at faster than 10 or 15 mph or in maintaining the siding to a higher speed than that. When power switches and movement by signal indication is introduced, the nature of train meets changes. Movement by signal indications makes possible the "running meet" in which neither train stops. In order to make maximum use of the signal control and power switches, sidings need to be much longer than the trains they are accommodating. If the switches at the ends of the siding allow 30 mph, ideally the siding will be long enough to allow the entire train to pull in at 30 mph then stop afterwards. For two trains arriving simultaneously at the meeting point when the siding is the same as the train length the running meet will consist of slowing both to 10 mph or less to ensure stopping short of the other end of the siding. When the siding is on the order of two miles long, exceeding train length by several thousand feet, a meet in which both trains continue moving at 20 mph or more is possible.
The absence of signals on a siding with power switches limits the effectiveness of the signal indication control of trains as well as the usefulness of the higher speed switches usually associated with such operation. The trains generally must reduce to 20 mph or less when entering the unsignaled track. When a siding is two or more typical train lengths with the intention of accommodating two trains at the same time, dividing the siding and the main track between the siding switches into two separate signal blocks can increase the effectiveness of the signaled siding. Once the first train has passed the intermediate signal, the second train may continue on signal indication instead of stopping at the signal governing movement into the siding, receiving authority from the Train Dispatcher to pass the stop signal, then proceeding at restricted speed.
The length of sidings and the speed at which trains can operate through them determines the amount of delay involved in meeting trains. The "headlight meet" will generally delay both trains as they slow to 30 mph or less depending on the length of the siding. When a train that must not be delayed is involved, the delay to other trains that is caused by slow speed or short sidings can be significant. A train pulling into a siding at 15 mph and stopping just to clear might consume 15 minutes more between sidings than a train continuing down the main track. To avoid delay to the important train, it must arrive over 15 minutes before the opposing train or remain at the previous siding. The delay will be on the order of 30 minutes if the sidings are 10 minutes apart. When the design of the siding allows trains to enter at 30 mph then stop, a train need only arrive 5 or fewer minutes ahead of the opposing train to avoid delaying it.
Copyright 1998 Thomas A White
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