Linespeed at Portcreek Junction (in Portsmouth)

[SWRtrainfan]

I was just casually browsing the forum and happened to find out about why the linespeed of the Portsmouth Direct line drops very briefly at Buriton Tunnel despite no significant change in curvature, something I'd been curious about for a while having driven the line a lot in Train Simulator (the reason being that the tunnel isn't wide enough for 23m stock, in this case 444s, and applying it to 450s as well because the cabs inside are too similar to those of 444s).

That then got me thinking about the other obvious permanent speed restriction on the line that isn't down to curvature, the drop from 85 to 60 then suddenly back to 85 just before Hilsea on the down. And similarly the drop from 80 to 50 then back to 85 just after Hilsea on the up. This speed restriction occurs over a bridge crossing the water, so if I were to guess, is it because the bridge is too weak for higher speeds? Or is it something else?

Thanks

 

[Snow1964]

Possibly dynamic loading on a bridge.

However sprinter SP, or multiple unit MU, etc differentials are often used at these, because lighter axle load trains put less load on the bridge.

The other main reason is restricted clearance, theory is that at lower speeds less likely to be as far sideways (forces pushing train outwards, within as far as springs / suspension allows). This movement on the springs up, down, sideways is usually referred to dynamic envelope, and train must not go outside this to avoid touching something. (Although Network Rail seems to allow vegetation to encroach in places)

 

[swt_passenger]

Off the wall suggestion here, although I think it’s most likely the bridge condition.

A possible dynamic restriction where the lines pass through the historic opening through Hilsea Lines, the northern fortification of Portsea Island. When the railway first opened I believe there was a closable barrier used overnight.

 

[hexagon789]

It was originally a blanket 70 through Buriton tunnel.

When the 442s and HSTs appeared down that way, it became an unusual reverse differential of WES/HST 40 over 70, meaning 442s and HSTs were restricted to 40 but other trains could do 70.

This is due to restricted clearance for 23m stock and the potential of the extra body sway at 70mph impacting on the kinematic envelope for 23m vehicles.

At some point Network Rail reduced it to a blanket 40, to negate the potential for drivers of stock of both 20m and 23m bodyshells forgetting which length they were in and exceeding the "safe" speed for 23m stock.

The present limit is 40 through the tunnel for ALL trains: