X Crossing v Switch Diamond Requirements

[Right Away]

I am wondering as to what factors dictate whether an X crossing or switched diamond crossing be installed. Are there constraints relating to speed over the crossing and more obviously the geometry (angle) of such?
Thanks for any info.

[Wickham Green too]

The constraint of cost is probably most pertinent. 

[keefer]

Switched diamonds are used when the route speed is over 40mph and/or the crossing angle is shallow enough (i.e. less than 1:8) to cause large gaps in the crossing components of a diamond.

Using switched rails eliminates the large gaps.

Previous topic here:

https://www.rmweb.co.uk/topic/19804-switched-diamonds/

 

Edited July 30, 2023 by keefer
Info

[The Stationmaster]

3 hours ago, Wickham Green too said:

The constraint of cost is probably most pertinent. 

That's why there was a major move towards replacing switch diamonds with 'ladder'  junctions on the face of it lookimng more complex but far x cheaper and easier to maintain.

[Southernman46]

Agree with the above - for example, Some years ago the original switch-diamonds at Hither Green coming off the Sidcup Loop (893 / 894 points in old London Bridge speak) were replaced like-for-like with plain obtuse crossing arrangement however the actual associated interlocking etc. was left in the signalling system as removing this would have overcomplicated the job no end.

 

Personally I hated SD's - the only 2 I ever had the mis-fortune to be responsible for were at Addlestone Junction on the SR. The damn things would fail as soon as the weather got a bit warm when blades would expand & the rodding for each of the SD ends would interfere with each other. It was the ONLY place in my PW area where the NR "let's paint everything white 'cos we can't do our HWP's properly" capitulation solution got used as mitigation 🤦‍♂️

[jim.snowdon]

This is a recurring subject and Keith Norgrove had it well described in this post 

in 2012.

 

Crossing angle is critical in diamonds as with the two obtuse crossings almost opposite each other correct, the principles of conventional check rails do not apply. Wheelset guidance is governed by the length of the immersed flange, ie that which is below rail level, being greater than the length of the flangeway gap. The latter increases as the crossing angle decreases, with a practical limit of 1:8. Shallower than that and switched diamonds, or moveable elbows in GWR-speak, become the only option. They are more complicated and more expensive to install and maintain, and for a long time railway track designers would go to considerable lengths to adjust the alignments of double junctions so as to engineer a crossing angle at or below the 1:8 limit. Those were also the days of mechanical operation and setting up the drives and detection would not have been easy. That changed with the general adoption of power operation and electrical detection and interlocking, but even then they are not highest on the list of Signal Engineers preferences. But, in conductor rail territory, diamond crossings, fixed or switched, have advantages over crossover ladders in terms of gapping risk.