AMain Track is a track that must not be occupied without a permissionto do so. Other tracks areSecondary Tracks. A Track used for meetingor passing (overtaking) trains is called aSiding. Sidings can beMain or Secondary tracks.
AStation is a named location on the railroad, listed in thetimetable.Europeans please note: A station need not be more thana trackside sign!
Switches (Turnouts) may beDual Control Switches, i.e. they areoperated remotely from CTC. Dual Control Switches may be operated by handlocally but this is mostly done during CTC failures.Spring Switchesare spring loaded in their normal position. They can be trailed from theother "reverse" track but needs hand operation if a train is to enter "reverse"track.
Restricted Speed is a speed low enough to enable the train tostop in half the visible range, short of stop signal, other train, wronglyset switch or track defect. Normally limited to 15 or 20 mph, dependingon the railroad.
APermissive Signal is a signal that shows "Stop then Proceed"or "Restricting" as its most restrictive indication. Normally indicatedby a number plate and or a "G" plate on the mast. AnAbsolute Signalis a signal that shows "Stop" as its most restrictive indication. Indicatedby absence of number plate on mast.
The signals described in this document are allBlock Signals,i.e. they govern trains entering the followingBlock. ABlockis the line between two consecutive Block Signals.Distant Signalsare signals on the approach to signaled territory, governing the approachto the first Block Signal. A Distant Signal does not indicate the conditionof the track between the Distant Signal and the Block Signal. A DistantSignal is identified by a plate with the letter "D" on the mast.
Track Bulletinsmay at any time modify the operation type.
In the descriptions below, the following symbols are used. Please refertext for an explanation of terms:
There is of course a set of rules to let a train enter CTC territorywhere a signal is not provided (say, from a spur) and to get trains movingin case of signal failure.
Signals in CTC territory is a mixture of controlled and automatic signals.
Below is an example of a single track CTC line, which would be listedasCTC in the timetable. The example shows how the dispatcher hasauthorized train movements by setting up train routes (light green lines).The eastbound train is to enter the first siding and meet the two westbounds.The CTC system only sets up train routes when it is safe to do so, otherroute requests are stored for later execution (blue lines). Automatic Signalsare usually permissive and will allow the train to proceed at RestrictedSpeed (yellow line). Thus the first westbound will be permitted to proceedfurther as soon as the eastbound is in the siding. The second westboundautomatically has permission to follow the first westbound to the nextcontrol point (east end of leftmost siding).
In this example the Sidings are under CTC, meaning that they are tobe regarded as Main Tracks (see discussionbelow). Though this is the most common nowadays, it is not always the case.Itshould be noted that this is not always the case. The siding itself maybe a Secondary Track and trains be allowed to enter the siding only onsignal indication "Restricting".
CTC xMT simply means that CTC is in effect and that the line has x MainTracks. Thus the example below is a line that would be listed asCTC2MT.
It will be noted that the siding areas in thefirst CTC example are in fact CTC 2MT sections on a CTC line. Traditioncalls for this type of line to be identified as CTC but with a note inthe timetable that also the sidings are under CTC. The same goes for sidingsin CTC xMT territory. The timetable thus expresses the main characteristicsof the line regarding to traffic operation. Extra long sidings, that arereally used as a second main track, on a CTC line, will be listed as CTC2MT sections in between CTC sections.
Today the Train Order System is obsolete and has been succeeded by systemspermitting train movements by radio. At least 3 such systems are in operationin North America, namelyTrack Warrant Control (TWC),DirectTraffic Control (DTC) andOccupancy Control System (OCS).
TWC:A Track Warrant is a permission to occupy main track betweentwo specific points. The points must be clearly identifiable and most oftenstation names are used, though mileposts may also come in handy.Detaileddescription of Track Warrant Control .
DTC:The main track in DTC territory is divided into named blocks,for example called "Alturas", "Canby", "Ambrose", "Perez" etc. A trainthe gets permission to occupy one or more blocks, for example block "Alturas"or blocks "Alturas" through "Perez".Detaileddescription of Direct Traffic Control .
OCS:For my further study.... (Please someone enlighten me onthis!)
Though these 3 systems differ, they basically do the same job. In thisdocument I will primarily use TWC as the Occupancy Permission System inthe examples, though most is also valid for the other systems. Detaileddescription of TWC and DTC will follow as my time permits, as these twosystems are the ones I know well enough to explain. I hope also to be enlightenedon OCS and other(?) Occupancy Permission Systems in the future.....
Applied in its basic form, an Occupancy Permission System has the solepurpose of ensuring thaton any given section of main track there isat no time more than one train. The Occupancy Permission System doesnot require any signals to accomplish this and may be used alone. Thistype of operation territory is often referred to asDark Territory.Below is shown a dark territory in which TWC is used, listed asTWCin the timetable. The traffic situation is equivalent to the CTC exampleabove:
The eastbound train is to meet the two westbounds at the first siding.The dispatcher has decided that the eastbound is to enter the siding andthe westbounds to use the main. The eastbound and the first westbound willtherefore both get permission to go as far as the west switch of the siding(illustrated by the dark green line), and the eastbound will be instructedto "clear main track". TWC allows to speed up the meet by issuing anotherTrack Warrant to the westbound permitting it to proceed westwards afterthe arrival of the eastbound train. Similarly the eastbound may get anearly Track Warrant, allowing it to proceed once the second westbound haspassed.
The second westbound can only be issued a Track Warrant permitting itto go as far as the first westbound is known to in the clear (MP XX inthe example). As the first westbound proceeds, the second westbound canbe issued new Track Warrants. Since issuing a Track Warrant is time consumingthis system is clearly not suited for dense traffic.
The Double Track (DT) variation of the TWC scheme is used to createlocations where easy meets may take place. Since all switches in dark territoryare hand thrown, trains need to stop to reverse or normalize a switch.Normalizing a switch after a train has left a siding has become a troublesomeand time consuming matter after the caboose (and thus the rear brakeman)disappeared from the trains. One often used and obvoius remedy is to installspring switches. Though this eliminates trouble when exiting a siding,it still requires someone to reverse the switch to enter the siding andsomeone to normalize the switch after the train.
Alternatively the railroad may decide to have the spring switches linedfor separate tracks instead, thereby effectively creating a short sectionof double track:
Besides eliminating the need to hand-throw switches entirely, this arrangementalso allows the trains to pass the location at more than restricted speed(since both tracks are now main tracks). Still, the rule of at most onetrain on a given section of main track is valid, since our line now hastwo main tracks.
The rulebook states that on double track, trains must keep to the rightunless otherwise instructed. So, a track warrant not specifying which trackto use, implies use of right track. The Track Warrant may instead statewhich track to use and thus overrule the right hand running. The designation"DT", instead of "2MT", really only serves to define how the tracks arenormally used.
ABS on Double Track means that each track is only signaled for trainstraveling with the current of traffic. The ABS ensures the safety for trainsrunning with the current of traffic (dark green and brown lines). ABS/DTis typically found between sections of CTC or CTC xMT where traffic requirementsdo not warrant the extra expense of bidirectional signaling. Trains canbe safely overtaken on sidings in ABS/DT territory without the need fora formal Occupancy Permission System. Rules, often combined with specialLeaveSiding signals, ensure that the train in the siding does not enterthe main track in a way that will compromise safety (cyan line). Trainson such lines are only informally told to, say, take siding to be overtaken.In some cases the dispatcher may leave it to the trains to decide, reportingtheir positions on the radio, on the best location for overtaking.
Where the ABS/DT section is very long, it becomes impractical not tohave crossovers between the main tracks. Even though these crossovers arehand-thrown, they are invaluable to keeping the traffic moving when trackwork and other causes necessitates to close one track down. To be ableto handle trains running against the current of traffic on parts of theline only, an Occupancy Permission System is introduced as an overlay tothe ABS/DT. The trains are in normal operation issued a permission to runthrough the whole line and as such the situation is similar to the ABS/DTtype of operation, including the informal way of instructing trains totake siding.
TWC on TWC/ABS/DT lines is relaxed to ensuring that eitherall trainson a given section of main track travel with the current of traffic, oronly one train occupies the line.
A single track ABS system is signaled for movements in both directions.Following the definition of the task of an ABS system, a single track ABSsystem is strictly not required to protect opposing trains(!). In realitymost single track ABS systems provide an almost complete protection ofopposing movements but some situations may result in a low speed collision.An exception to this is theAbsolute Permissive Block (APB) system(see below) which in some implementations provide full safety for opposingtrains. Most APB applications do, however, sacrifice a little safety foroperational flexibility and therefore also needs an occupance permissionsystem overlaid.
The purpose of single track ABS its to allow trains to follow each otherin close succession. The presense of ABS allows the TWC to be relaxed toensuring thatall trains on a given section of main track travel inthe same direction.
Description of single track ABS systemused on the Southern Pacific .
As mentioned above, someAbsolute Permissive Block (APB) systemsprovide full safety for opposing as well as following trains. An APB systemmay be technically completely identical to an ABS system, the only differencebeing that signals at the end of siding areas are absolute (Red means "Stop(and stay)") instead of permissive. It is therefore not necessary to useany Occupancy Permission System with the APB, though the Occupancy PermissionSystem may be a handy tool for the dispatcher to control traffic. Pleasenote that two trains meeting face-to-face at restricted speed, as indicatedon the Main Track along the left Siding, is perfectly safe, though generallynot very practical...
Some railroads use only informal dispatching in APB territory and thistype of operation is listed asABS orAPB.
A train moving within Yard Limits under a Block Signal indication (i.e.in ABS or CTC territory) more favorable than "Approach" does not have tomove at Restricted Speed. Trains encountering a Block Signal indicating"Approach" within Yard Limits must slow to Restricted Speed immediately,if possible from that Block Signal.
The requirement for Restricted Speed after encountering "Approach" isprobably an extra precaution to avoid mishaps when the train is movingslower within Yard Limits. Within Yard Limits there is a much higher probabilityof another train or engine wishing to enter the Main Track in question.Even though these trains must comply with rules for entering ABS or CTCtracks, the risk of misunderstanding each others intentions, and misjudgingwho's moving under signal indication and who's moving at Restricted Speedis much higher than outside Yard Limits.
Movement on Double Track against the Current of Traffic is however notpermitted exept by Track Warrant, Track Bulletin or permission from Yardmasteror other authorized employee. While Restricted Speed should make such amove safe, this rule will in practical terms force trains to move withthe Current of Traffic, lowering the probability of mishaps....
Before making an entry in the Block Register, it must be checked thatprevious entrien are complete, i.e. that there is no other train in theBlock Register Territory. If a second train needs to occupy the Block RegisterTerritory, the first train must be informed of the joint occupation. Allmovements must then be made at Restricted Speed.
Text, HTML: Carsten S. Lundsten.