points and crossings

[L&Y Robert]

On the diagram posted by MatkyT http://www.townend.me/files/oxford.pdf (v4) on the Oxford Station thread -
At the North end we see an array of crossovers proposed (Yes, I know they're there already):

Down Loop to Down Main
Down Main to Up Main
Up main to Up Loop
Up Loop to Bicester

WHICH

- present a lunatic serpentine route across the LAYOUT (not "Job") if traffic had to go from the down platform, say, to Bicester. I appreciate that they together present an "All possibilities" set of options, but they take up length ("We've GOT length") but with possiblities of the kind of derailment we saw not long ago at Camp Hill, Birmingham. "Turning to the right, now the left, now the right, now the left again". Phew!

This sort of arrangement is common in the UK - I remember being slewed across once, twice, three times to get into Darlington recently, same again to get out! And there was a similar arrangement at Swindon at one time, both ends of the station. But at Bristol, there used to be a layout with extensive use of double slips, all taken out and replaced in some scheme of rationalisation some years ago.

But looking at pictures of practice in, say, Germany, they use single and double slips to do this sort of thing. Why are single and double slips so despised in UK? And why do we go to such lengths to avoid crossings? - single lead junctions etc.

 

[edwin_m]

Diamonds and slips are considered a maintenance headache so simple turnouts are used wherever there is enough space to do so. The same may be true in other countries, but their stations tend to have more platforms and therefore more complicated throats. I believe new materials for crossings may make diamonds less of a problem than they used to be.

Single lead junctions are gradually on the way out. As well as the safety issue (which is largely eliminated by TPWS) they reduce capacity, because a flat junction is used most efficiently if two trains on the same route can pass on the junction. Which on a single lead they obviously can't.

 

[D1009]

Isn't it normally the case that single and double slips will have a much lower speed than normal pointwork?

 

[civ-eng-jim]

Isn't it normally the case that single and double slips will have a much lower speed than normal pointwork?

Standard slips have speeds of 20 - 25mph. They're really only used where space is a premium, depots and station throats, so line speed isn't so much an issue.

 

[dysonsphere]

I seem to rememder Guildford had a ladder of slips before the tunnel entrance guess they wernt a problem as I think everything stopped there so speed restrictions didnt matter.

 

[MarkyT]

What you really want to avoid are switched diamonds, (aka 'elboes', 'angles, etc). Fixed diamonds aren't too much of a problem and can be accommodated in low speed layouts where the crossing angle is is quite large, like slips as mentioned above. Fixed diamonds have no moving parts so there are no actuation mechanisms or detection limit switches to fail and delay trains. Higher speed diamonds have smaller crossing angles, and eventually as the junction speed rises further the angle becomes so small that the flange gap is too large for safe and comfortable transit. At that point the simple fixed crossing elements must be replaced by a pair of very closely spaced tip-to-tip moving switches, These are just as complex as two conventional turnouts placed tip to tip from signalling point of view, yet they are significantly and notoriously more difficult to maintain in adjustment, often causing frequent detection failures. In new and redesigned layout they should be avoided if possible, but in some cases they are unavoidable due to space constraints. Where those constraints do not apply conventional wisdom results in a series of crossovers forming double and single lead ladder type junctions. That has been best practice for many years in UK and also applies elsewhere, especially for faster junctions. Using slips instead of a ladder also results in greater complexity with reliability implications, so again where the ladder will fit at the design speed it is usually the preferred solution.

 

[telstarbox]

Can someone explain 'single lead' and 'double lead' to a layman (i.e. me)?

 

[The Planner]

Single track and double track junction basically.

 

[najaB]

Can someone explain 'single lead' and 'double lead' to a layman (i.e. me)?

There are some diagrams on Wikipedia.