Articulated Carriages: Maximum Length?

[Clansman]

As the title states, this is something that has me thinking for various reasons.

I understand the basics of articulated carriage designed. But not being from an engineering background, I am unaware of the constraints faced by this approach when it comes to carriage length. Naturally I imagine - as any basic research on this shows - carriage length is inevitably reduced since there is less support for carriage weight that traditional bogies would bare.

The Eurostar and TGV carriages show that with Jacob's bogies on both ends you can achieve 18.7m. With full bogies one one end and Jacob's bogies on the other, you can achieve just under 22m.

The constraints using a design like that of Talgo's with one-set of wheels, I imagine would constrain this further. From what I can see from their off-the-shelf design, they can achieve 14m per carriage. Stadler Flirts typically are shorter, so I can't see them as being a useful gauge for this question on that basis.

So in essence, is it known what the maximum length could be for articulated carriages with today's technological knowledge given Talgo and the TGV designs are decades old?

 

[zwk500]

I suspect the bigger constraint is not the material strength to span the wheelbase but rather the impact on the structure gauge when cornering - particularly platforms.

 

[Bletchleyite]

I suspect the bigger constraint is not the material strength to span the wheelbase but rather the impact on the structure gauge when cornering - particularly platforms.

I'd say so, yes.

In essence the maximum length of a coach on two Jacobs bogies is the maximum bogie centre distance you could have if the same vehicle was conventionally bogied. If you see what I mean. If it's longer, the middle of the coach will start whacking platforms.

As a guide fully articulated FLIRT intermediates are typically 16m (same as a Pacer vehicle). You can go longer but if you do you need to go narrower.

 

[Irascible]

With the reduced number of axles per vehicle it's possible there are dynamic constraints too, but I couldn't say for sure without designing something

 

[hwl]

So in essence, is it known what the maximum length could be for articulated carriages with today's technological knowledge given Talgo and the TGV designs are decades old?

"technological knowledge" has nothing to do with it, structure gauge and overthrow on curves has everything to do with it. All relatively simple trigonometry.
The longer the distance between bogie pivot centres the narrower the vehicle has to be given a fixed structure gauge.

The maths (for the GB railway network) is covered in appendix A, page 110 of GERT8073 Issue: 4.1 Date: December 2022

Specifically for the for the lower structure gauge:
Ti = (A*Ni - Ni^2 ) / 2R ,
To = (A*No + No^2 ) / 2R

where;
• Ti is inner throw in metres.
• To is end throw in metres.
• A is bogie centres in metres.
• Ni /No is the distance in metres from the nearest axle or bogie centre.
• R is curve radius in metres

Extra margins are added to Ti and To for track curvature radii below 360m and 200m.