Why is the linespeed 85 mph?

[Bevan Price]

I never have understood why the class 33s were built as 85 mph locos, I can only assume this was specified to suit the Southern routes they were destined to work.

In the steam era, the Southern Region had a 85 mph maximum anywhere on the region. I think that Scottish Region had a similar (but lower) all-over blanket speed restriction (75 mph I think)..


I think on some other lines, a 70 mph limit became the norm because all passenger trains were 1st generation dmus, all of which were limited to 70 mph.

 

[asylumxl]

and on one rural line I can think of - the risk of a passenger sticking his / her head out and nutting a bridge.

Best thing I've read all day.

 

[Dieseldriver]

85mph linespeeds feature very heavily on the routes I sign. Must admit, it always strikes me as an odd number!

 

[notadriver]

85mph linespeeds feature very heavily on the routes I sign. Must admit, it always strikes me as an odd number!

Yes I was thinking about your line and few others in the south west where 85 mph seems to be the upper limit but this may have been explained historically by Bevan Price earlier.

 

[JB25]

Love motoring down beyond Purley at 90mph. One thing I always find slightly odd... When you are going through Merstham tunnel at 90 it feels pretty normal to be honest, when you exit the tunnel it quite literally feels like you are flying a rocket. Weird how the differential in the surroundings make you feel like you are going faster when you are doing exactly the same speed.

 

[Cherry_Picker]

Is all this just because we are conditioned to speed limits being posted in multiples of 10 on our road network?


I'd always wondered why the roads are rounded in multiples of 10. The other country which still uses miles to measure their road network, the USA, are happy to post speeds in multiples of 5. I've never really had a problem with the main line using multiples of 5.

 

[talltim]

mph is an arbitrary measurement of velocity anyway

 

[edwin_m]

So is everything else, except perhaps the ratio of speed to the speed of light, which would need bigger signs because of the number of decimal places.

 

[met331]

Somewhat surprisingly, "HST" also includes 158s, 159s and Turbostars.

but more surprisingly doesnt include the 185 due to their weight and braking capabilities

 

[edwin_m]

I can understand why the 185 is too heavy to take the "SP" differential speeds but not why it can't take the "HST" ones, as its axle load is surely less than that of a HST power car. Maybe it's simply that nobody has asked, because there are few if any sections where 185s regularly operate that also have HST differential speeds.

 

[Eagle]

Also 390s are prohibited from using HST differentials, if they ever come across them. In fact the only electric stock that can use HST speeds is 91+Mk4 and 373s.

 

[northwichcat]

Until the 350s were made 110mph capable there wasn't any stock that could run at 110mph, so why should 85mph be any more strange?

 

[TheKnightWho]

Until the 350s were made 110mph capable there wasn't any stock that could run at 110mph, so why should 85mph be any more strange?

Class 90..?

 

[Bevan Price]

Yes I was thinking about your line and few others in the south west where 85 mph seems to be the upper limit but this may have been explained historically by Bevan Price earlier.

The 85 mph limit may have included a considerable safety margin. Many steam locos were not fitted with speedometers until the 1950s - some of the smaller locos probably never received speedos before they were withdrawn at end of regular main line steam working in 1968.

 

[martybabes]

Buriton tunnel was 70 but because of the curvature of the tunnel and the increased length there was a chance of the 442s hitting each other if 2 passed in the tunnel so they were restricted to 40 through it.

Do they get wider as they go faster then?

 

[Eagle]

Do they get wider as they go faster then?

No, but they do lean over more.

Centripetal force

 

[martybabes]

No, but they do lean over more.]

I understand that

Centripetal force[/URL

I didn't understand any of that

 

[edwin_m]

Not really to do with centripetal force, it's mainly the swaying and other motion of the train on its suspension. This will increase at higher speeds.

 

[A-driver]

Not really to do with centripetal force, it's mainly the swaying and other motion of the train on its suspension. This will increase at higher speeds.

I believe that is why the tunnels in the east grinstead line are 60 whilst the sections either side are 70 or 85 if I remember correctly. It's also why the up slow line in potters bar tunnel drops from 75 to 65, the swaying movement could cause 2 trains to come into contact if overtaking whilst the down lines run through the new 1950s(?) tunnel which are wider.

I seem to remember that when they tried clearing 319s for the east grinstead line they ran a 377 through oxted tunnel at 60mph with a polystyrene block built around the top to make it the size of a 319. When it came out the other end it was covered in dents and scratches so they couldn't be cleared for 60mph.

 

[ChiefPlanner]

I seem to remember that when they tried clearing 319s for the east grinstead line they ran a 377 through oxted tunnel at 60mph with a polystyrene block built around the top to make it the size of a 319. When it came out the other end it was covered in dents and scratches so they couldn't be cleared for 60mph.

A very simple and practical way of checking "clearance" !!!!
A