Motto: “I don’t see the code anymore. All I see is blonde, brunette, redhead, negative, adverse.” Cypher

Disclaimer: This is not a design guidance. Don’t believe everything you read online.

LandXML?

LandXML is a non-proprietary data standard on the Extensible Markup Language (XML) format, developed for engineering data exchange. A LandXML file can contain civil and survey data elements such as points, profiles, surfaces, materials and many others. 

For railway track engineering, the format is often used to interchange alignment design information for design and construction purposes.

One of the track parameters included in the file format is the Cant. It was the last of the railway track alignment parameters included in the LandXML schema in version 1.1, in 2006.

Normal, Positive, Adverse or Negative Cant?

Cant on conventional railway track is most of the time applied on curves by lifting the outer rail, to compensate for you know what. Let’s call this type of cant “normal”.

Some railway administrations prefer to correlate the sign of the cant with the sign of the curvature. The real curvature.

A right-hand curve is “positive” and will have “positive” cant if the cant is normal for that curve, left side rail lifted.

A left-hand curve is “negative” and will have “negative” cant if the cant is normal for that curve, right side rail lifted.

When, due to the constraints of the design, the cant must be applied by raising the inside rail of a curve, that is called “adverse” cant or “negative” cant. The most common occurrence of this case is on the canted contra-flexure diverging route of a curved turnout.

This is the only one I will refer to after this section. Remember this. I’ll call this type of cant “NEGATIVE”.

Sometimes the cant must be applied by lowering the inside rail of a curve. This is also called “negative” cant, a common encounter on complex light rail or tramway projects. In this case the normal cant is … negative.

Obviously, this case of “negative” cant is different from the “adverse”-synonym “negative”.

The urban design environment sometimes forces the designer to have the track canted for no railway related reason whatsoever, to keep the light rail track in good coordination with the road. The cant is then neither “negative” nor “adverse”, but, indeed, I dare to say, ”perverse”.

Urban railway design confronts the designer with various monstrosities. These are just a few:

• cant on straight (easy?); is that “negative” or “positive”?
• cant switching from inside to outside rail on one curve;
• cant applied in equal or variable proportions to inside and outside rails;
• cant due to steep vertical gradient (for example a track crossing a road with a 3% gradient ends up with 45mm “perverse cant” if not compensated in the road profile);
• steep cant gradient due to track not matching road geometry (never cross a roundabout).

To keep this simple, let’s ignore combining any of these with branching routes at junctions.

Very often the practical solution to all these design challenges is to have for each rail a separate vertical profile. Oh, what a shock when you see for the first time the vertical alignment of the canted rail! (Note to self – this is a good subject to blog on).

Even when the rails are designed with separate vertical alignments, the designer must still check the unusual design against the cant rules. Cant is a design twist. Too much twist derails.

How does LandXML deals with so diverse types of cant?

In short, so much better than most of the design software we use today.

A few details can be found in the LandXML v1.2 schema documentation.

The Cant LandXML element has many parameters but the ones relevant for applying the various types of “adverse”/”negative” cant are the following:

• appliedCant. The “appliedCant” is a required double that is the applied cant.  This value is expressed in millimeters or inches depending upon the units.”
• Rotation Point. The “rotationPoint” is an optional string that defines the rotation point.  Valid values are “insideRail”, “outsideRail”, “center”, “leftRail” and “rightRail”.

For conventional railway track the rotation point for cant is the inside rail. Other options are available and most welcomed.

This parameter defines the entire cant alignment and cannot be set for individual cant stations. However … never mind.

• Adverse. The “adverse” is an optional Boolean that indicates whether the cant is adverse.

Boolean? Fancy math wording. That means “true” or “false”.

• Curvature. The “curvature” is a required enumerated type.

Eh! “Curvature”! Probably one of the most used technical words on this blog.

The meaning of this parameter called in the LandXML Cant schema “curvature” is not the alignment curvature. If this would have been related to the actual curvature, then that would have been a horizontal geometry parameter.

“Curvature” in the Cant LandXML element means in fact “track cross-sectional rotation direction to apply a positive cant value” (my most inspired definition). This is independent of the horizontal geometry and of the real alignment curvature.

This Cant “Curvature” can be “cw” (clockwise) or “ccw” (counterclockwise). Rotation direction, not curvature! My God, what’s wrong with these people?

When is the cant “NEGATIVE” in LandXML?

Let’s see first three examples of when it is not “NEGATIVE” but other types of “negative” I mentioned above.

Let’s say we need a “NEGATIVE” cant of 50mm on a right hand curve.

1. Set “appliedCant” = – 50mm. No other change.

This will lower the outer rail by 50mm relative to the vertical alignment level. This is a negative cant. But not the NEGATIVE we want.

2. Set “Adverse” = “true”. No other change.

This will lower the outer rail by 50mm relative to the vertical alignment level. This is a negative cant. But not the NEGATIVE we want.

Same as case 1.

3. Set “appliedCant” = – 50mm and “Adverse” = “true”. No other change.

This will lift the outer rail by 50mm relative to the vertical alignment. This is normal cant.

Yes, the key is the “curvature”. It sets how the cant is applied, totally independent of the horizontal alignment. With it, you can have cant on straight and adverse/NEGATIVE cant.

If “curvature” is set to “cw”, the sleeper is rotated clockwise relative to the “rotationPoint”. Left rail will be lifted.

If “curvature” is set to “ccw”, the sleeper is rotated counterclockwise relative to the “rotationPoint”. Right rail will be lifted.

This works independently of the horizontal alignment curvature.

For our example, to get a NEGATIVE cant for a right hand curve, Set “appliedCant” = 50mm and set “curvature” = “ccw” to lift the right rail, the inside rail of the curve. If you want to keep this rail canted, keep “curvature” = “ccw” no matter how the alignment curvature varies.

This is how the cant can be set NEGATIVE in LandXML.

Easy!

Remember, this is not a design guidance. And it is on purpose confusing, perhaps even wrong. Check for yourself. You’ve been warned!