Railroad Rules, Signalling, Operations:

Direct Traffic Control

by Carsten S. Lundsten. Last updated 26-07-1998.


The DTC Block

Movement Authority
     Directional Authority
     Restricted Authority
     Work and Time Authority
     Multiple DTC Authorities in the Same DTC Block

     Changing Authority
     Releasing DTC Block Authority
A DTC Example


Direct Traffic Control (DTC) is a verbal authorization system defined by the General Code of Operation Rules (GCOR), used to authorize trains to occupy Main Tracks. DTC can be used as a stand-alone safety system in unsignaled territories, or can be supplemented with Automatic Block Signaling (ABS) to increase flexibility and traffic capacity. DTC is as far as I know used by Union Pacific and CSX, on the former Southern Pacific, Chicago and North Western and Seaboard Coast Line. Direct Traffic Control on the former Chicago and North Western lines was quickly replaced by Track Warrant Control after the Union Pacific takeover, as TWC is the systems used by Union Pacific. If is not unlikely that DTC will also disappear from the former Southern Pacific lines within the next few years. TWC is used on the other western railroads and is a more flexible - but also more complex - system.

Direct Traffic Control divides the line into fixed DTC Blocks. Trains are authorized to occupy specific DTC Blocks. DTC rules specify fixed wording for messages concerning the grant or release of DTC Blocks. The train crews must keep a written note of the DTC Blocks their train is authorized to occupy. The train releases the DTC Block Authorities after it has cleared the DTC Blocks.

The DTC Block

A DTC line is divided into fixed DTC Blocks. The DTC Blocks are named, typically after a Station at one end of the DTC Block. The extension of each DTC Block is defined in the Timetable and is also marked by lineside signs.

A line with regularly spaced Sidings will usually be divided into DTC Blocks extending from one Siding to another. At some locations, however, the line between two Sidings has been subdivided into 2 or more DTC Blocks. Below is shown an example of a DTC line:

The Stations Anna and Danby have a Siding for meets. Bolo is just an intermediate Station while Coyote Jct is the junction of another line.

DTC Block boundaries at Anna and Danby are at one end of the siding, in this example the east end. Placing the boundary at one end of the Siding helps speed up certain meets, as will be shown later. The DTC Block boundary at Bolo it at the Station sign. Many stations like Bolo have once had a Siding, and therefore the DTC Block boundary may still be where it was in the days of the Siding. At Coyote Jct it is most practical to place the DTC Block boundary so as to facilitate trains going on or off the branch without occupying the DTC Block to the West. In the example the DTC Blocks are named after the Stations at the east end of the Blocks, thus defining the DTC Blocks "Anna", "Bolo", "Coyote", "Danby" and "Eve".

Movement Authority

A DTC Block Authority is issued in the following steps, usually using radio: A DTC Block Authority cannot not be considered valid until the dispatcher says "<Train>, that is correct.".

Directional Authority

Directional Authority authorizes a train to occupy one or more DTC Blocks, moving in a specified direction. A train having been issued a Directional Authority is not allowed to move backwards in the DTC Block. But since a DTC Block is only the Main Track, a train may set back into a spur or siding.

To illustrate how a Directional Authority is issued, we'll use the example below. A train (7241 East) has Authority in the DTC Block Anna only, and the Dispatcher is to issue Directional Authority to DTC Blocks ahead:

To give Directional Authority in one DTC Block, the following wording is used:

After receiving "That is correct", 7241 has Directional Authority in DTC Block Bolo as well:

To give Directional Authority in two DTC Block, the following wording is used:

To give Directional Authority in more than two DTC Blocks, the following wording is used:

Restricted Authority

In dark territory a train may be issued Restricted Authority to follow behind a train granted Directional Authority to that same DTC Block. Movement must be made at Restricted Speed. Restricted Authority may only be granted one train in a DTC Block. Restricted Authority may only be granted after the Dispatcher is has assertained him/herself that the train ahead has passed the point where the train given Restricted Authority will enter the DTC Block. When the train ahead has released the DTC Block in question, the requirement for Restricted Speed may be canceled, converting the Restricted Authority to a Directional Authority.

Restricted Authority is typically used to allow a train to pull out of a Siding after being overtaken. If the train has to line a switch back behind itself, it can pull out on the Main on the Restricted Authority. In most cases the train ahead has cleared the DTC Block before the second train is ready to depart.

To give Restricted Authority in one DTC Block, the following wording is used:

Canceling the requirement for Restricted Speed is done using the wording: After receiving "That is correct", 7241 has Directional Authority in DTC Block Bolo as well.

Work and Time Authority

Work and Time Authority permits a train to occupy the DTC Block(s) and move in either direction. Several trains may be present in the DTC Block under Work and Time Authority, in which case movements must be at Restricted Speed. A second train may not be issued Work and Time Authority to a DTC Block before the train already issued a Work and Time Authority is informed that the DTC Block will be jointly occupied. As the name suggests, a Work and Time Authority is time limited, i.e. the train must have cleared the DTC Block(s) before that time expires. The DTC Block(s) cannot, however, be considered clear until they are released by the train.

Work and Time is issued by the dispatcher using the wording:

Work and Time may be granted in several DTC Blocks, using similar wordings.

Work and Time may be issued only if:
  • The DTC Block is clear.
  • Or (in ABS territory only) the DTC Work and Time Authority will not take effect before the arrival of a train running on Directional Authority in the DTC Block. 
  • Or the DTC Block has been allocated under Work and Time Authority to another train which is informed that the DTC Block will be jointly occupied.

Multiple DTC Authorities in the Same DTC Block

Only one DTC Authority may be issued issued in the same DTC Block, except in the following cases:
  • In ABS territory, multiple Directional Authorities may overlap in the same direction, as the ABS will protect following trains from each other.
  • In Dark Territory, as specified for Restricted Authority, to follow another train still in the same DTC Block. After the first train releases the DTC Block, the Dispatcher may upgrade the Authority to a Directional Authority.
  • At meets, when Directional Authority will not take effect before arrival of an opposing train. The DTC Authority must include the words "...after the arrival of <train>".
  • Under Work and Time Authority when trains are instructed to move at Restricted Speed.

Changing Authority

If it becomes necessary to change the DTC Block Authority previously granted to a train, a new Authority is granted in the specified manner. After the "That is correct" message, the previous Authority to the DTC Block(s) in question becomes void.

Releasing DTC Block Authority

When a train has cleared a DTC Block, the DTC Block must immediately be released to the Dispatcher, unless the Dispatcher has specified that immediate release is not necessary.

A train may release a DTC Block only when the entire train has cleared the DTC Block. If a train clears the DTC Block by entering a Secondary Track, the switch must be lined back to the Main Track before releasing the DTC Block.

To release one DTC Block, the following wording is used:

Similar wordings are used for the release of more than one DTC Block, following the pattern for issuing Authorities.

A DTC Example

The following is an example of how Direct Traffic Control may be used to control an unsignaled, single track, line. The example line is similar to what has been shown above, with the two sidings Anna and Danby, though Danby is now controlled by CTC. In between are the stations Bolo and Coyote Jct. The timetable text would read something like:

DTC in effect between <Somewhere West> and Danby.

CTC in effect between west switch Danby and <Somewhere East>.

This means that the siding at Danby is under CTC, while the siding at Anna has manual switches.

Two trains are to meet at Anna. The trains will be called 585 East (Eastbound) and 5032 West (Westbound). 585 East will be arriving first to the meet and is to take Siding. Since the switches are hand-thrown, the 585 East has to line the switch back after pulling into the siding. This means leaving the Conductor behind to line the switch back, after which he/she will have to walk up to the locomotive.

The DTC Authority for 585 East, from a DTC Block somewhere west could read:

585 East would also be instructed to take siding at Anna and, since there's little other traffic, that it need not release its DTC Blocks before clearing the Anna Block.

5032 West is somewhere beyond Danby and has not yet been issued a DTC Authority to go further than Danby. Only the signal into the main track at Danby has been cleared but even if the signals were clear through Danby, 5032 West would not be permitted to depart Danby without a DTC Authority:

As 5032 West approaches Danby it is time to issue a DTC Authority. Since 5032 West is an ordinary train and no other trains are ahead, 5032 West is issued a Directional Authority to the meet at Anna:

5032 West is also instructed that it need not release its DTC Blocks before clearing the Bolo Block. When the DTC Authority has been issued, the dispatcher clears the signals out of Danby:

585 East has how arrived at the west switch at Anna and prepares to head into the siding. 5032 West is approaching Danby:

585 East is now in the siding and releases its DTC Blocks:

With 585 East in the clear, 5032 West can be permitted to continue past Anna. The dispatcher issues a further DTC Authority to 5032 West:

5032 West now has DTC Authority in all DTC Blocks between Danby and Somewehreelsewest. Since other traffic doesn't require the DTC Blocks behind 5032 West, there is no reason to release them one by one.

The dispatcher also issues a new DTC Authority for 585 East:

"...after arrival of 5032 West" suspends the DTC Authority until 5032 West has cleared the east switch at Anna:

As soon as 5032 West clears the east switch at Anna, 585 East's Directional Authority comes in effect. 5032 West releases the traversed DTC Blocks:

585 East's Directional Authority comes in effect even before 5032 West has released the DTC Blocks to the Dispatcher.

After 585 East has pulled out of the Siding, our poor conductor will once more have to line the switch back and walk up to the locomotive before 585 East can get on the move again. The dispatcher has set up a new meet for 585 East at Danby, but this time all controlled by CTC:

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Text, Images and HTML: Carsten S. Lundsten.