Aim - this section provides an overview of couplers, and seeks to describe an "optimal" coupler setting based upon the current setup for Open Rails (OR) WAG and ENG files.

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Advanced Coupler

Coupler Breaking Force

Sample code for Advanced Coupler

Useful References


Over the years, with the development of railways, a number of different types of couplers have been used to connect adjoining wagons together. The equipment that connects the couplings to the rolling stock is known as the draft gear or draw gear.

The majority of the coupler types allow a certain amount of slack to occur between wagons. So in railway terms, slack action, is the amount of free movement of one car before it transmits its motion to an adjoining coupled car. Some amount of slack is required in the coupling to allow the coupling to be flexible enough to go around curves on the railway. Slack can also be very helpful in starting a train. If the train is stetched then the locomotive needs to pull the full load of the train at once, and if the train has a high starting resistance, then the locomotive may not have sufficient power to start. To overcome this the train can be 'bunched' up, and thus when the train starts the locomotive is only starting one car at a time until the slack is taken out of the coupling system, thus the starting resistance of the train is substantialy reduced as the cars start to move. Slack can also work against the train driver as it allows individual cars to move independently of each other, and thus they can end up travelling at different speeds to each other. If the speed difference is high enough between individual vehicles, then the forces produced in the coupler between adjoing cars maybe high enough to break the coupler.

Coupler Forces Displacement Diagram

The action of the various types of couplers can be described on a Force vs Displacement diagram as shown in the two example figures, above which are described more fully in the he article Simulated comparisons of wagon couplersystems in heavy haul trains. The diagram on the left is for an older style Draw Hook and Buffer connection, whereas the diagram on the right is from a more modern Auto coupler connection. The Auto Coupler presents a more symmetrical graph as it has similar characteristics regardless of whether the coupler in in tension (pulling) or compression (pushing). The Draw Hook and Buffer connection is more asymmetrical in appearance as when the coupling is in tension (pulling) the Draw Hook is more rigid, and has less slack then an Auto Coupler, wheereas when it is in compression (pushing) the Buffers tend to have a more cushioned effect similar to an Auto Coupler.

Open Rails uses this type of approach to define how the coupler responds during train movements. Two options are provided in OR, a 'Simple' version which is the original OR version, and an 'Advanced' version. The setting of the Advanced version is described below, and the Simple version should be the same as it has always been.


Advanced Coupler

Note: Advanced Coupler functionality is currently only available in the MG version of Open Rails.

The Advanced Coupler is modelled on a three zone displacement model as described in the following diagram.

OR Coupler Forces Displacement Diagram

Thus to specify a coupler it is necessary to describe the displacement distances for each zone, as well as the applicable forces at these points. Also as the coupler can have different characteristics when it is tension or compression, each of these different areas need to also be specified.

The parameters required to specify the coupler in accordance with the above diagram are as follows:
ORTSTensionStiffness ( a b ) - coupler forces, where a = coupler force at end of Zone 2, and b = coupler force at end of Zone 3.
ORTSTensionR0 ( a b ) - coupler displacement distances, where a = zero length (adjusts distance between cars), and b = displacement distance of Zone 1.
ORTSTensionSlack ( a b ) - coupler displacement distances, where a = displacement distance of Zone 2, and b = displacement distance of Zone 3.
ORTSCompressionR0 ( a b ) - coupler displacement distances, where a = zero length (same as value above), and b = displacement distance of Zone 1.

ORTSBreak ( a b ) - Coupler breaking force, where a = force at end of Zone 2, and b = force at end of Zone 3.
CouplingHasRigidConnection ( a ) - defines coupler as rigid (very small amount of slack), where 0 = flexible coupler, and 1 = rigid.

Note: All values should be +ve equivalent (including compression values as OR adjusts them as appropriate).

In setting up the slack in the wagon there may need to be a trade off between the spacing between the wagon, when it is bunched, and when it is strecthed, as OR doesn't currently support couplers which move with slack.

A sample code for specifying a coupler is provided in the Sample code for Advanced Coupler section. To see how the Advanced Coupler performs, and for a working code copy, look at the relevant Demonstration Activity.

To view the coupler forces and other coupler related paramter performance in Open Rails, refer to the extended HUD, FORCES INFORMATION by pressing the Shft-F5 key multiple times until the relevant HUD screen is found.


Coupler Breaking Force

If the force on the coupler exceeds this force then the coupler will break (requires "Break" option to be selected in OR Option Menu). The force on each of the couplers in the train can be identified in the extended HUD. The coupler force will vary depending upon the number of wagons connected to the coupler, ie the load, and will also vary as the train negiotiates hills or curves, etc which increase the load force. In driving the train the driver needs to be conscious of not putting excessive loads (forces) on the coupler.

For NSW operational practice, typical coupler breaking forces (or Draw Capacity) are somewhere between 0.15MN (150kN) and 4.45MN (4500kN), as summarised in the table below, which shows a selection of the wagons, and locomotives from the documents in the Useful Links section below. Refer to the Draw Capacity column. Similarly values for UK practice are shown in one of the other document, including screw and draw hook couplings.

Coupler Type

Draw Capacity (MN)


Coal wagons - modern permanently coupled (Eg PHEH)


Bar type connection

Coal wagons - bogie (Eg BCH)


autocoupler, metal body

Water Gin (Eg L229)


Rail Car - Modern style (Eg XPT)


Diesel - Modern (Eg 5000 Class)


Special coal haulage locomotive

Diesel - Branch Line (Eg 48 Class)



Diesel - Mainline (Eg 442 Class)



Passenger - Modern style (Eg ARL)


Steel body

Passenger - Modern style (Eg SBN)


Steel body

Passenger - Older style (Eg FO)


Wooden body

Steam locomotive - Modern (Eg 3801)


Steam locomotive - Older style (Eg 2705)



Sample Code for Advanced Coupler

The following code sample has been set up to match the Miner M-901E Draft Gear.

Typically the lines shown in red text are the only ones that would need to be changed on individual wagons.

Comment ( Open Rails Advanced Coupler )
Coupling (
Spring (
ORTSTensionStiffness ( 1.29e6N 9.63e7N )
ORTSTensionR0 ( 0cm 2.5cm )
ORTSTensionSlack ( 6.35cm 7.747cm )

ORTSCompressionStiffness ( 1.29e6N 9.63e7N )
ORTSCompressionR0 ( 0cm 2.5cm )
ORTSCompressionlack ( 6.35cm 7.747cm )

ORTSBreak ( 1.8e6N 1.8e6N )
CouplingHasRigidConnection ( 0 )


Force Conversion

Convert long Tons (UK) force to Newtons.

tonsf (UK) to N:
Force (tonf):       
Force (N):     Force (kN): Force (MN):


Useful References

Establishment and verification of three-dimensional dynamic model for heavy-haul train-track coupled system

NSW Locomotive and Rolling Stock Data

The Evolving Coal Wagon by Ross Peter Golotta

Structural Requirements for Drawgear and Buffers on Railway Vehicles (Superseded UK standard)