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Monday, September 30, 2013

CSX: Where TWC and DTC Collide

Like most North American Class 1 railroads CSX is an amalgamation of various predecessors. Not only are there the Seaboard and Chessie System components (themselves the result of mergers), but the former Conrail and Louisville and Nashville territories and several smaller lines such as the Rechmond, Fredericksburg and Potomac and Monon. Each were incorporated with their own rulebooks and signaling practices and until recently CSX continued to maintain multiple rulebooks across its network. In 2007 CSX switched to a brand new unified rulebook, but in order to avoid any drastic change in operating practices many of the previous flavours of rules were simply combined into a single document. Apart from the signal rules (which have sections for Chessie, Conrail and Seaboard aspect systems) the section which most heavily reflects this sort of conglomeration is the one concerning Track Warrant Control (section 5, page 9, if you want to follow along.)

Before the Conrail merger in 1999 CSX used Direct traffic control on all of its unsignaled lines. DTC makes use of fixed blocks with movement authority provided from the dispatcher to the train directly via radio communications (hence the D in DTC). This was in contrast to previous systems where trains would follow a predetermined timetable and any changes would have to be given to crews via block operators or lineside telephones (aka Indirect Traffic Control). When Conrail trritory was incorporated those lines used the Form D Control System or DCS. This was similar to DTC it referenced stations and interlockings as valid block limits (with a block limit sign being a form of station), but don't let the D in DCS fool you as DCS is a Track Warrant system, not a DTC system.  Track Warrant Control not only allows for movement authorities to be given between traditional block limits, but also to any whole milepost or switch.  Moreover Track Warrants can be used for other things besides train movement authority and usually end up combining several forms, like speed restriction updates, into one. While in theory this could have just meant a change in paperwork, mucking about with safety critical systems takes care so CSX decided to make the new TWC system backwards compatible with the old and therefore split it's new TWC rules int two sub-sections TWC-DTC and TWC-DCS to accommodate the differences between the original DTC and DCS systems.

While TWC is more versatile, DTC is simpler and more efficient at doing the things it does.  Instead of trains needing to write out new or amended track warrants, DTC blocks cab be released as they are cleared and then immediately given to other trains. Still, it is clear that CSX's preference lays with the TWC-DCS rules as they immediately went about ripping out all of the old DTC block signs wherever it was most practical like on signaled main lines where DTC used to substitute during signal outages.  DTC references were also largely removed from the employee timetables indicating that the sign removal wasn't just a cosmetic change.  Still, there remain a few locations where DTC remains in effect, mostly on unsignaled secondary lines where use of DTC is more efficient than having crews fill out full Form D's.

Say Goodbye to old CSX DTC Block Signs Like These
Where DTC will be most difficult to dislodge is in the circumstance of bi-directional ABS without traffic control, known under NS as Rule 271 or out west as TWC-ABS.  One might see bi-directional automatic block signals as an indication that a line as been equipped with CTC. However as spelled out in its name, CTC is a system for Traffic Control, not block separation. Traffic control means that when a train is given a route into CTC territory a flow of traffic is established such that opposing movements are not permitted to enter that line. Failure to establish a flow of traffic can result in Mexican standoffs away from passing points or certain race conditions that can cause a head on colision. Because CTC requires logic beyond simple automatic block occupancy to establish a traffic flow back in the day it was more than some lines wanted to deal with. However the advantages of ABS with its broken rail, hand switch point detection and flexibility for following movements could not be ignored. While not common on CSX it does appear occasionally, with the former Monon Railroad main line being perhaps the best known example with the line retaining the same system of operation even after the semaphores were removed.  The new system, known as TWC-DTC (ABS) (!) is at least slightly more clear than the old Rule "120-132 (243-246)" label. 

To accommodate both its traditional and signaled flavors, there are several types of DTC block authorities that can be given. The first is Absolute Block, which has the standard definition, but allows trains with such authority free use of the block in both directions. The next is Clear Block. This grants authority in a single direction guaranteeing the block free of obstructions, but allows say a train in a siding to proceed in the opposite direction on its own DTC authority once passed by the conflicting movement. The third is Occupied Block, which like the name implies allows a train into a block with an obstruction. This is not intended to be used for permissive operation, only the case where one train needs to assist another or pickup equipment left unattended on the tracks. Finally there is a fourth type of DTC permission, Proceed Block. This grants trains the authority to proceed governed by fixed signal indication. This is the only type of authority given in TWC-DTC (ABS) blocks and this rule unlocked the mystery of why such a system exists in the first place back when I first encountered it several years ago.  While all other railroads use full length Track Warrants on bi-directional ABS territory CSX has allowed this situation to remain the province of DTC even as DTC is abandoned elsewhere. 

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