What are HST speeds applicable to?

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Class 170101

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From Oxford Parkway to Bicester (and beyond to Bletchley) is 75/HST 100mph. It will see 10tph (both ways) when you add in 2tph to MK and 1tph to Bedford; 11tph with freight.
Why are we creating HST speed boards for this route? The only section ever likely to have seen a HST will be Oxford to Bicester (and High Wycombe). The section beyond Bicester to Bletchley will (almost) never see one.

Surely they should IEP boards? Whats special about a using a HST Speed Board that isn't done by something more modern?
 
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swt_passenger

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Why are we creating HST speed boards for this route? The only section ever likely to have seen a HST will be Oxford to Bicester (and High Wycombe). The section beyond Bicester to Bletchley will (almost) never see one.

Surely they should IEP boards? Whats special about a using a HST Speed Board that isn't done by something more modern?
Voyagers (and certain other stock as the next post now lists) use HST speed differentials as well. It certainly isn’t ‘created’ just for what is usually called an HST.
 

Domh245

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Why are we creating HST speed boards for this route? The only section ever likely to have seen a HST will be Oxford to Bicester (and High Wycombe). The section beyond Bicester to Bletchley will (almost) never see one.

Surely they should IEP boards? Whats special about a using a HST Speed Board that isn't done by something more modern?

HST is a standard speed differential across the network that applies to all manner of stock (IC225, 158, 159, 168, 170, 171, 172, 175, 180, 220, 221, 222, HSTs, 373s, 80x)
 

Andyjs247

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185s have been touted as an interim solution for EWR. But they cannot use HST differentials, which would seem to be a potential problem. I am also curious as to the reason for the differential speeds here given that the line was rebuilt only 5 years ago to modern standards and is fairly straight.

I get that there wouldn’t have been any pressing reasons then for eg a Class 185 to run on this route at 100mph, but why it is not simply 100mph for all traffic?

Given that Chiltern do run Class 68+Mk3 at 100mph here I would not imagine there being many practical reasons why 185s could not run at the HST speed differential, other than it perhaps never having been requested or specifally cleared.
 

hexagon789

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Why are we creating HST speed boards for this route? The only section ever likely to have seen a HST will be Oxford to Bicester (and High Wycombe). The section beyond Bicester to Bletchley will (almost) never see one.

Surely they should IEP boards? Whats special about a using a HST Speed Board that isn't done by something more modern?
It's a legacy thing, HSTs were among the first disc braked stock bar some commuter EMUs and so were among the earliest to have superior braking capability. Later trains match that so can use the speeds unless the axle loading prohibits it.

There's no need to replace it, you just apply it to successive stock, I don't really see why the IETs need their own differential when they can use an existing one?


I am also curious as to the reason for the differential speeds here given that the line was rebuilt only 5 years ago to modern standards and is fairly straight.
Axle loading? Signal spacing? Could be a number of things.


Given that Chiltern do run Class 68+Mk3 at 100mph here I would not imagine there being many practical reasons why 185s could not run at the HST speed differential, other than it perhaps never having been requested or specifally cleared.
I think it would have to be a specific exception given they can't use HST differentials ordinarily, so it may well simply be that the 68s can use it as they run there and it was thought worthwhile to get an exception, but 185s haven't so have never been cleared.
 

LNW-GW Joint

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A further curiosity is that the category "HST" includes stock limited to 90mph, but not new 125mph speed stock like 397, or even 100/110mph stock like 350/755 etc.
These are classified within the "MU" category, which seems to handle all speeds on NR.
Proper high speed lines (HS1, and no doubt HS2 when it comes), not being part of NR, also have their own rules.
ETCS and cab signalling may eventually change all this, with no need for lineside speed signs.
 

SynthD

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There are specific exceptions in the sectional documents, I think Chiltern’s southern section has a few.
 

hexagon789

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There are specific exceptions in the sectional documents, I think Chiltern’s southern section has a few.
Yes, there's a few exceptions on most regions.

For instance, 158/159s were only allowed to use HST differentials within about the last decade or so, but on older Scotland Sectional Appendices there is a note that HST speeds apply additionally to Class 158s in Scotland.

Bound to be other similar things
 

The Planner

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It is down to Appendix C braking curves or 9%g braking capability in passenger trains. For those who want to get bogged down in the standards, it is GKRT0075 and GMRT2045.
 

hexagon789

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It is down to Appendix C braking curves or 9%g braking capability in passenger trains. For those who want to get bogged down in the standards, it is GKRT0075 and GMRT2045.
Is that what they call it instead of the W125 braking curve?
 

norbitonflyer

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Axle loading? Signal spacing? Could be a number of things
As I understand it, it's usually where the speed limit is determined by signal spacing (rather than curvature, weight limits etc). The adage "your permission to speed is your ability to stop" - which is why some lines have higher speed limits uphill than down.
 

hexagon789

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As I understand it, it's usually where the speed limit is determined by signal spacing (rather than curvature, weight limits etc). The adage "your permission to speed is your ability to stop" - which is why some lines have higher speed limits uphill than down.
I remember reading about the HSTs as introduced having to be limited to 120 where otherwise one would expect a full 125 limit because of the uphill gradients giving less benefit than for 100mph running as regards the braking distances for some signals
 
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