Motto: Tolle lege.

Most of the railway track design standards around the world are presenting the particular issue of the reverse transition.

All these standards are insisting on keeping a constant rate of change of curvature – a constant A. By doing this in fact the design will include a single continuous transition between the two opposite circular arcs. I’m sure you remember, dear reader, the two reverse infinite loops of the clothoid presented previously here.

The curvature diagram of this alignment will be something like this:

Nothing spectacular till now. This is common sense alignment design engineering. That is why most of these standards, when presenting the reverse transition, don’t insist too much on this curvature rule but on something else, of similar importance but more subtle … an “orphan” rule of cant design.

What’s that?

Let’s see some snapshots from a few standards, perhaps you will notice too this orphan cant (superelevation) design rule:

1. Railway Track Design Standards, Norms and Design Handbooks

Hmm!
“Reverse transition should have a constant rate of change of curvature … Levels should be set out as shown in the diagram …”

Wait!
Lift?!
What lift?!
… let’s see some other standards first …

Linke schiene – left rail
rechte schiene – right rail
Uberhohungsrampe – cant ramp (transition)

2. Light rail and tramway track design standards

Have you noticed the mystical triangle and the lift of the reverse point shown in all these standards?

3. Technical books

And this is not all. Lifting the reverse point of the transition is not only a railway (or tram) track design rule. You will find the same principle also in the road design standards.

4. Road design standards

When a reverse curve is used on a road project, and the radii are requiring superelevation, the runoff of the superelevation is designed to lift the reverse point:

It is written in the books and standards … but can any of the design software we use do this?

5. Design software documentation.

Both Bentley Track alignment design software, Railtrack and MX, are able to automatically design the cant over the reverse in the way prescribed in the standards:

But not only them. I’ve seen a demo on it using Ferrovia and designed so many such alignments with ProVI – a very good railway track design software, by the way.

Why?

Installing the cant in this way is keeping the track centerline on an almost constant level throughout the reverse transition in opposition with what is seen as the normal way of applying the cant – as two separate transitions, without the lift.

The difference between the two ways of applying the cant over the reverse transition can be seen in the animation below:

Which one seems better for you?
Why did I called this “the orphan rule”?
You tell me, dear reader … why?
How significant is this?

The exact significance will be explored in a future post here and in more detail, probably, in a future article published in the Journal of the Permanent Way Institution.

References

Here is just a short list of the standards and books presenting this orphan rule:

• B50 (2004). Oberbau – Technische Grundsatze Linienfuhrung von Gleisen. OBB (Technical principles. Track alignment.  Austrian track national standard)
• Bentley Railtrack Help (2015). Civil Help SELECT Series 4. Help file for Bentley Railtrack V08.11.09.84x Maintenance Release.
• Bentley MXRoad Suite Help (2012). MXROAD Suite Online Documentation. Help file for Bentley MXRAIL 08.09.04.XX Release.
• Esveld, C. (2001). Modern Railway Track. MRT Productions.
• Departement de l’Environnement, des Transports et de l’Agriculture .Transports Publics Genevois  (tpg)
• IRPWM (2004) Indian Railways. Permanent Way Manual. Ministry of Railways. India.
• Instructia 314 (1985). Instructia de norme si tolerante pentru constructia si intretinerea caii. Linie cu ecartament normal. INCERTANS. Romania. (Instruction of norms and tolerances for track construction and maintenance. Normal gauge lines – Romanian national track standard).
• M 22-01.12 (2015). Design Manual. Washington State Deportment of Transportation (WSDOT). page 1250-11.
• Morgan, J.C. (2009). British Railway Track. Volume 1. Track Design. Part 2 Switches and Crossings. Permanent Way Institution.
• NR/L3/TRK/2049/mod02 (2016). Track Design Handbook. Issue 13. Module 2 – Mathematics. Network Rail. United Kingdom.
• RIL 800.0110 (1999) Richtlinie – Netzinfrastruktur Technik entwerfen – Linienführun. Deutsche Bahn. (Directive – Infrastructure Network Technical design – Lines. German track alignment design standard)
• SANRAL (***). Geometric Design Guidelines. South African National Roads Agency Limited (SANRAL). page 4-16.
• TCRP Report 155 (2012). Track Design Handbook for Light Rail Transit. Second edition. Transportation Research Board. Washington -page 3-35.
• TPG (2016) Directives technicques pour tramways. Chapitre4. Geometrie de la Voie.
• НКЖИ (2010). Инструкция за устройство и поддържане на горното строене на железния път и железопътните стрелки. Sofia, Bulgaria (Instruction for track construction and maintenance of track superstructure, plain line and switches and crossings. –Bulgarian national track standard) – page 84.