Curves, and how they are measured

[Ian J.]

Given that the OP has freed this topic to roam in the wild, are preserved railways obliged to perform these calculations and check alignments accordingly, or are they exempt under their LRO's by keeping speed under 25mph?

I would hazard a guess that even at 25 mph if there weren't any transitions between straights/curves then the ride would be pretty rough, so I imagine they would implement them even if only for the comfort of their passengers.

[Edwin_m]

The effect of transitions also depends on the wheelbase and end overhang - if there is no transition in the track then there is in effect a "virtual transition" of the same length as the bogie centres of the vehicle running on it, or the axle spacing for a four-wheeler.  The latter was illustrated very well on the Combino trams in Amsterdam, which have no rear cab.  Consequently the rear seats are on a long overhang from a very short wheelbase bogie which is moreorless rigidly attached to that section of the tram making in in effect a four-wheeler.  On every tight non-transitioned curve this overhang would whip round from its straight to its curved position in the short time it took the "bogie" wheelbase to travel onto the curve, and similarly at the return to straight track.  Soon after that the Combinos all had to be recalled, as it turned out the people at Siemens had used heavy rail accelerations when designing the structure and the traction package was at risk of falling through the ceiling due to metal fatigue.

[Derekstuart]

A little while back I asked a question on another thread about how turnouts work on canted track, and was fortunate enough to get a very detailed explanation from Mr. Wynne. It was only then that I started looking further into how geometry is calculated and it is quite incredible just how many calculations are required now to produce curves, and the rules that govern 'you can do X but not if also doing Y, but that rule can be ignored if doing Z'.

 

I know most of it is not really that relevant for model purposes, but nonetheless it is interesting to learn.

 

Incidentally, for anyone who doesn't know, turnouts on canted track can use thicker chairs to raise rails to match up with other canted track or to return it to straight. What I still don't understand is how they behave when the diverging road is on the OUTSIDE/UPPER side of the track- you need to reduce the height, not raise it. Any takers?

[martin_wynne]

What I still don't understand is how they behave when the diverging road is on the OUTSIDE/UPPER side of the track- you need to reduce the height, not raise it. Any takers?

 

The timber under it is tipped down at an angle from the horizontal ("raked"), and then thicker bases are used under the other rails to bring them back to the required level.

 

Martin.

[Derekstuart]

Thank you Martin. As ever you are very generous with your time and information.

 

That was what I first assumed from your other explanation about increasing the chair thickness, but then I changed my mind and thought it couldn't be right. I should have stuck with my first thought.

 

Many thanks.

[martin_wynne]

Hi Derek,

 

Sorry that last reply was a bit terse. I was interrupted in the middle of writing it.

 

regards,

 

Martin.

[Derekstuart]

Martin,

 

thank you, but I didn't take it that way anyway. You of all people have no need to apologise for being terse (which you weren't). I was asking a bit of a silly question to which I already knew the answer; add that to the hundreds of other questions that appear here and elsewhere every day...

 

I don't specifically target you with questions as I know you are only one person and your time is not un-limited, but it is always very reassuring to get the answer from you.

 

Thanks again for all your advice.

 

EDITED for grammatical inexactitudes.

[dp123]

I was still measuring curves with 20m of fishing line and a ruler in 2009/10 when attached to the PWay Tech Office and doing platform and structure surveys. The real skill is keeping the line taut without snapping it, never mind the calculations (naturally chalked up on the platform face or bridge).

[GordonC]

A little while back I asked a question on another thread about how turnouts work on canted track, and was fortunate enough to get a very detailed explanation from Mr. Wynne. It was only then that I started looking further into how geometry is calculated and it is quite incredible just how many calculations are required now to produce curves, and the rules that govern 'you can do X but not if also doing Y, but that rule can be ignored if doing Z'.

 

I know most of it is not really that relevant for model purposes, but nonetheless it is interesting to learn.

 

Incidentally, for anyone who doesn't know, turnouts on canted track can use thicker chairs to raise rails to match up with other canted track or to return it to straight. What I still don't understand is how they behave when the diverging road is on the OUTSIDE/UPPER side of the track- you need to reduce the height, not raise it. Any takers?

 

somewhere to look for an example of that might be Carstairs station. The main line track is canted diverging to the right (south) while the line to Edinburgh branches off on the outside and has a 15mph restriction. Also a crossover to the opposite platform for Edinburgh to Glasgow trains.

[Derekstuart]

Yes there is a video on youtube of a Brush 4 heading out towards Edinburgh.

 

You don't really notice it when onboard, but there is a fair bit of twisting there- granted the telephoto lens makes it look worse than it really is.

 

Certainly when coming off the up slow that does seem to be a hell of a severe cant gradient (again- telephoto lens).

 

I've been over that crossing more times than I could ever remember- Not the same since they put those damned wires up to Edinburgh. BTW I don't suppose you know exactly where the wires stopped, do you please? I can't locate it exactly on the map but ISTR it is after the first curve.

somewhere to look for an example of that might be Carstairs station. The main line track is canted diverging to the right (south) while the line to Edinburgh branches off on the outside and has a 15mph restriction. Also a crossover to the opposite platform for Edinburgh to Glasgow trains.

[keefer]

page about carstairs from jim howie's site (via wayback machine): http://web.archive.org/web/20130530142050/http://www.jhowie.force9.co.uk/carstairs.htm

[Derekstuart]

Thanks Keefer. What a very interesting site.

 

EDITED.

[scotcent]

 

Quite  alarming  when  you  see  it.  This  train  has  come  from  Edinburgh  and  will  have  to  negotiate  a  crossover  to  get  to  the  down  platform.

 

Allan  F