Differential Speeds

[Step3 Star]

With regards to the Differential Speed Boards for HST, MU and SP and the signal spacing distance for the track involved to stop the train.

I believe that with say a 125HST/100mph speed board the standard signal spacing would be for 100mph.

With a MU or SP differential speed board with say a 90MU or SP/75mph would the standard signal spacing be for 90mph or 75mph?

Thank you in advance for any help given.

 

[yorkie]

With a MU or SP differential speed board with say a 90MU or SP/75mph would the standard signal spacing be for 90mph or 75mph?

See this thread from 2018:

Someone else will no doubt correct me, but the reasoning behind the categories is slightly different:

HST relates to braking performance (Appendix 3 of the relevant signalling standard)

MU, DMU and EMU relate to route availability (principally axle loading), and braking performance (Appendix 2 of the relevant signalling standard)

SP relates to route availability (principally axle loading), and braking performance (Appendix 3 of the relevant signalling standard)

No idea about CS, although I remember some fruity discussions about permitted speed with EWS when they bought them.

 

[Annetts key]

When HSTs were introduced, the disc brakes that they were fitted with substantially improved the stopping distances compared to the conventional braked trains that operated on the network. A far better braking curve therefore allows for a higher speed running compared to conventional braked trains, with the same distance between signals for both.

And during testing of the Great Western ATP the braking performance was further confirmed with HSTs during test runs apparently stopping in shorter distances than the team expected.

Although also keep in mind that on the lead up to the introduction of the HSTs on the Western Region, BR did carry out various track and signalling improvements. I don't know all the details, but bull head and jointed flat bottom rails were replaced with CWR/LWR (and sleepers replaced with concrete types). Many of the conventional track circuits were replaced with ASTER audio frequency types (that don't need IRJs). I don't know if there were any alterations to signals or signal spacings though. And I don't know what happened on other regions.

 

[baz962]

I just always want to know why 222's can only do 75 round Manton and other trains 90

 

[hexagon789]

I just always want to know why 222's can only do 75 round Manton and other trains 90

Because it's 75 / SP 90 and 222s can't use Sprinter differentials?

 

[baz962]

Because it's 75 / SP 90 and 222s can't use Sprinter differentials?

Well yes. But why as in what actually is the problem , the reasoning. Surely a 222 could handle those speeds.

 

[hexagon789]

Well yes. But why as in what actually is the problem , the reasoning. Surely a 222 could handle those speeds.

Too heavy I'd assume.

SP speeds are based on braking capability AND axle load.

 

[Step3 Star]

Thank you for your replies.

 

[Hairyspotter]

There's a similar paradox where there's a 75/SP90 between Nuneaton and Leicester . 170s can run at the SP speed but 22x can't despite having similar RA and better braking capability.

 

[Step3 Star]

There's a similar paradox where there's a 75/SP90 between Nuneaton and Leicester . 170s can run at the SP speed but 22x can't despite having similar RA and better braking capability.

Would the signal spacing be worked out on the 90mph speed or the 75mph speed??

 

[Annetts key]

Would the signal spacing be worked out on the 90mph speed or the 75mph speed??

It's not just the speed. It's the combination of the traffic type (which normally means including freight on mixed traffic lines), the braking curve of the trains and any gradients. The signalling is normally designed for the worse case unless it's acceptable to have a differential speed restriction. Then the lower speed is the worst case. And passenger trains that have a better braking curve (better brakes) will have a higher permissible speed (as long as the track is suitable for this).

Alternatively the signalling can be designed for the passenger trains that have the better braking curve, but with the differential speed for all other trains reduced to comply with the worse case braking curve.

 

[edwin_m]

Classes 150-156 have tread breaks and I believe therefore only meet the traditional 7%g braking curve. These are also 75mph maximum and able to use SP restrictions, implying that SP at 75mph is based on 7%g stopping distances.

Classes 158 onwards are disc braked with 9%g braking and cleared for 90mph or 100mph. BR Regional Railways introduced a lot of 90mph SP differentials on the basis that 158s with better brakes could stop from 90mph within the existing 75mph signal spacing. Therefore SP at speeds above 75mph must be based on the 9%g braking curve.

 

[craigybagel]

Recently, to support the regular use of loco hauled services along the route, the Crewe - Shrewsbury line changed from 90MU/70 to 90/70. There was only one location along the route where the MU standards couldn't be met by a non MU passenger train (Harlescotte on the up, which is now 85/70 from the distant signal) - otherwise the standards were presumably already being met. Possibly because the line was relatively recently resignalled?

Around the same time, most of the MU differentials between Shrewsbury and Hereford were also replaced with passenger freight differentials, and with it being predominantly AB signalling this did result in a lot more changes, and several places where MU trains are now running slower than they did with the previous MU speeds.

 

[Step3 Star]

Thank you all for the replies.