SWR Portsmouth services diverted via Oxshott on 3/4 August

Deepgreen · 1 Aug 2026

The BBC News web site has a story saying that, owing to hot weather, from 1200-1900 on Monday 3 August and Tuesday 4 August, Portsmouth line trains between Guildford and Waterloo will be diverted via Clandon. It doesn't say why, only for service 'resilience'.. Does anyone know the reason? RTT does not show the diversions yet.

Some South Western Railway trains to be diverted due to hot weather

South Western Railway says some services will be diverted on Monday and Tuesday.

www.bbc.co.uk

South Western Railway (SWR) has said some of its services will need to be diverted during a continuing period of hot weather.

The train operator said services from Portsmouth Harbour to London Waterloo, will run via an alternative route through Cobham after departure from Guildford on Monday and Tuesday.

The changes will be in effect from 12:00 to 19:00 BST on both days, with the operator saying it will "help us provide a more reliable and resilient service".

SWR said online journey planners will be updated 24 hours beforehand and passengers have been advised to check before they travel.

The alterations follow the company's decision to halve the number of trains between Yeovil Junction and Exeter St Davids, as part of alterations on the West of England line to London Waterloo.

RailUK Forums

Bow Fell · 1 Aug 2026

I’d imagine its to protect the points at Woking Junction, so they're maintained in one direction to prevent any failures, protecting the South Western Mainline.

Harpo · 1 Aug 2026

Bow Fell said:
I’d imagine its to protect the points at Woking Junction, so they're maintained in one direction to prevent any failures, protecting the South Western Mainline.

That was called a key route strategy in the past although it was usually intended for severe winter conditions.

Oxalin · 1 Aug 2026

My understanding is the switch diamond at Woking Junction (where the Up Portsmouth crosses the Down Fast) struggles in high temperatures, with the expansion of metal putting it out of detection. Diverting Up Portsmouth services via Cobham means it can be kept in normal for the Down Fast.

moley · 1 Aug 2026

During one of the earlier heatwaves, up services crossed to the down line ahead of the junction then used P4 towards London. This forces some down services to P2 to avoid delays, or more common, just an acceptance of delays. It also puts additional stress on the crossings north of Woking with extra movements.

By switching up services to run via Clandon they have a, generally non conflicting, journey through P3 at Guildford and run just ahead of the stopping services. They rejoin at Hampton Court junction again with little conflict.

I noticed during the previous heatwave, the section was marked No-Go presumably to remind the signaller to route accordingly.

Deepgreen · 2 Aug 2026

Thanks for the responses. I don't recall this being done in the previous heatwaves this year, where the sun strength would have been even higher, but I may have missed it. It wasn't reported in news feeds anyway.

Harpo · 2 Aug 2026

Deepgreen said:
I don't recall this being done in the previous heatwaves this year, where the sun strength would have been even higher,

Is there a cumulative effect at play here with energy continually building up in assets and no respite where they might cool down?

slicedbread · 2 Aug 2026

The SWR website at https://www.southwesternrailway.com/plan-my-journey says

We have identified an issue with the signalling equipment during periods of warmer weather.

To help minimise service disruption, there will be alternations to services in the Woking area on Monday 3 and Tuesday 4 August.

Between 10:00 and 17:00, services from Portsmouth to London Waterloo via Guildford will be diverted via Cobham & Stoke D’Abernon.

The diversion will not increase journeys times. However, customers travelling to Woking should change at Guildford, for an hourly service to Woking.

Oxalin · 2 Aug 2026

Deepgreen said:
Thanks for the responses. I don't recall this being done in the previous heatwaves this year, where the sun strength would have been even higher, but I may have missed it. It wasn't reported in news feeds anyway.

It's happened on several occasions (and not just this year), although you're right this is the first time it's been picked up on by news outlets as far as I'm aware.
Typically it's only off peak, and generally not every service presumably due to route knowledge constraints

Deepgreen · 3 Aug 2026

Oxalin said:
It's happened on several occasions (and not just this year), although you're right this is the first time it's been picked up on by news outlets as far as I'm aware.
Typically it's only off peak, and generally not every service presumably due to route knowledge constraints

If it's not every working, surely that negates the benefits of leaving the points at Woking set? I notice, too, that RTT still has none of the diversions that I can see - perhaps they will just be VSTs tomorrow?

== Doublepost prevention - post automatically merged: 3 Aug 2026 ==

Having looked at RTT this morning I can see no workings diverted via Cobham today. Has the plan been abandoned?

Oxalin · 3 Aug 2026

Deepgreen said:
If it's not every working, surely that negates the benefits of leaving the points at Woking set?

It still reduces the number of movements - and potentially the remaining up services via Woking can be routed through P4 or P5 to avoid the switch diamond.

Deepgreen said:
Having looked at RTT this morning I can see no workings diverted via Cobham today. Has the plan been abandoned?

No amendments on RTT but they are on JourneyCheck, for example the 10:33 Portsmouth Harbour to London Waterloo is shown as not calling at Woking, with a vaguely accurate reason:

03/08/26 10:33 Portsmouth Harbour to London Waterloo due 12:22 will be diverted between Guildford and London Waterloo.
It will no longer call at Woking.
It will be delayed due to the diversion and is expected to be 26 minutes late.
This is due to a signalling problem.

Nammer · 3 Aug 2026

So the SWR original post said 'the diversion will not increase journey times' ... right?!?

moley · 3 Aug 2026

So the stopper which the diverted service almost catches is due at Surbiton at xx40 therefore the diverted service should be through Surbiton at xx42/xx43. It is ordinarily timetabled through at xx38. So 5 minutes delay yes, where us the other 20 coming from?

Deepgreen · 3 Aug 2026

Oxalin said:
It still reduces the number of movements - and potentially the remaining up services via Woking can be routed through P4 or P5 to avoid the switch diamond.

No amendments on RTT but they are on JourneyCheck, for example the 10:33 Portsmouth Harbour to London Waterloo is shown as not calling at Woking, with a vaguely accurate reason:

Isn't that unusual? Seems like a clear case for VST to me. Not only a different route used, but Woking missed as a station stop, so not just a minor alteration.

Oxalin · 3 Aug 2026

moley said:
So the stopper which the diverted service almost catches is due at Surbiton at xx40 therefore the diverted service should be through Surbiton at xx42/xx43. It is ordinarily timetabled through at xx38. So 5 minutes delay yes, where us the other 20 coming from?

Nowhere, it was only 6 minutes late in the end: https://www.realtimetrains.co.uk/service/gb-nr:L79619/2026-08-03/detailed#allox_id=0. SWR have a tendency to massively overestimate how long this diversion takes. In reality if it weren't for the stopper I'm pretty sure via Cobham is a minute or two quicker than via Woking.

Deepgreen said:
Isn't that unusual? Seems like a clear case for VST to me. Not only a different route used, but Woking missed as a station stop, so not just a minor alteration.

I don't think I've ever seen this particular diversion as a VSTP to be honest

Horizon22 · 3 Aug 2026

It's down to KRS (Key Route Strategy). Some routes will have differing "tiers" depending on the temperature thresholds which vary throughout the day. This will prevent certain points being used, restricting access for services in progressively worse patterns. This is supposed to be proactive before any failure of assets occurs.

For example on Western during roughly the same period, GWR are running the Didcot Parkway terminators non-stop from Reading to Paddington (missing Twyford, Maidenhead and Slough) so they don't need to cross from Main to Relief lines and vice versa. This is partly the weather and partly due to the poor performance of a number of the points anyway.

Deepgreen · 3 Aug 2026

Oxalin said:
Nowhere, it was only 6 minutes late in the end: https://www.realtimetrains.co.uk/service/gb-nr:L79619/2026-08-03/detailed#allox_id=0. SWR have a tendency to massively overestimate how long this diversion takes. In reality if it weren't for the stopper I'm pretty sure via Cobham is a minute or two quicker than via Woking.

I don't think I've ever seen this particular diversion as a VSTP to be honest

Ok, so I am wondering how this significant change to services would normally be shown on RTT?

moley · 3 Aug 2026

Presumably if they are not publishing a VSTP entry, then the published timetable of the day remains via Woking for these services therefore any delay on a journey to / from / changing at Woking qualifies for delay repay if more than 15 minutes delayed.

Deepgreen · 3 Aug 2026

moley said:
Presumably if they are not publishing a VSTP entry, then the published timetable of the day remains via Woking for these services therefore any delay on a journey to / from / changing at Woking qualifies for delay repay if more than 15 minutes delayed.

Given that the change is a planned one, not an emergency, this seems strange.

== Doublepost prevention - post automatically merged: 3 Aug 2026 ==

moley said:
Presumably if they are not publishing a VSTP entry, then the published timetable of the day remains via Woking for these services therefore any delay on a journey to / from / changing at Woking qualifies for delay repay if more than 15 minutes delayed.

I can't imagine how anyone wanting to get between Woking and Guildford could avoid being delayed by way more than 15 minutes. However, my main interest was how the signallers, etc., deal with the issue - are they just at liberty to re-route trains of their choice? Seems a little haphazard for a planned event.

Steddenm · 3 Aug 2026

Some SWR services from Portsmouth to Waterloo are being diverted today via London Road Guildford and the "new line" not calling at Woking.

Travelled this morning around 1030 from Portsmouth & Southsea and was told the train wouldn't stop at Woking. We were diverted between Guildford and Surbiton and delayed by around 20 minutes on arrival.

Was the guard that mentioned it was due to the heat at Woking.

wickham · 3 Aug 2026

Deepgreen said:
If it's not every working, surely that negates the benefits of leaving the points at Woking set? I notice, too, that RTT still has none of the diversions that I can see - perhaps they will just be VSTs tomorrow?

== Doublepost prevention - post automatically merged: 3 Aug 2026 ==

Having looked at RTT this morning I can see no workings diverted via Cobham today. Has the plan been abandoned?

Under these arrangements those up Portsmouth line services that are not diverted via Cobham run into Woking via the down Slow (Platform 5) or down Fast (Platform 4) both of which are reversable which brings them into Woking via different route and thereby not using the diamond crossing at Woking Junction.

Falcon1200 · 3 Aug 2026

Deepgreen said:
Seems like a clear case for VST to me.

If the diversion takes longer than the booked route, as seems to be the case, it will probably not be possible to create a valid new schedule, ie one which does not conflict with any other service. When I was involved in such things, a few years ago now, resulting delays in such cases were sometimes attributed to Schedule Error, even though this was unavoidable and not in any way the root cause of the delay!

Belperpete · 3 Aug 2026

Steddenm said:
Some SWR services from Portsmouth to Waterloo are being diverted today via London Road Guildford and the "new line" not calling at Woking.

Is this anything to do with the heat?

800002 · 3 Aug 2026

Belperpete said:
Is this anything to do with the heat?

Likely, yes.
If it avoids swinging points at busy intersections.
(Key Route Strategy to assist in keeping railway performing / operating)

Steddenm · 3 Aug 2026

Belperpete said:
Is this anything to do with the heat?

Yes, points and crossovers at Woking apparently.

wickham · 3 Aug 2026

Falcon1200 said:
If the diversion takes longer than the booked route, as seems to be the case, it will probably not be possible to create a valid new schedule, ie one which does not conflict with any other service. When I was involved in such things, a few years ago now, resulting delays in such cases were sometimes attributed to Schedule Error, even though this was unavoidable and not in any way the root cause of the delay!

In these cases, if diversions are "short notice" a VSTP schedule is not required to be put in - the diverted train just reports "off route" and is regulated "as best possible" by the signallers and controllers.
I have known Freightliner trains between Southampton and the north, booked via Oxford/Birmingham area, diverted at short notice (while they were on the run) via Woking, Old Kew, Wembley and the West Coast Main Line and they have picked up "off route" sucessfully. Those Eastleigh Freightliner drivers have amazing route knowledge !!

Horizon22 · 3 Aug 2026

Deepgreen said:
Given that the change is a planned one, not an emergency, this seems strange.

== Doublepost prevention - post automatically merged: 3 Aug 2026 ==

I can't imagine how anyone wanting to get between Woking and Guildford could avoid being delayed by way more than 15 minutes. However, my main interest was how the signallers, etc., deal with the issue - are they just at liberty to re-route trains of their choice? Seems a little haphazard for a planned event.

They will more than likely be given a service group (e.g. 1Axx) and told where that is going. Also the traffic management system on Wessex will be edited accordingly. Regulation will be done via "best endeavours" with some for of preference decided.

DelW · 3 Aug 2026

From RTT, up morning services ran via Woking p2 as normal. After about noon they either ran via the New Line or used p5 at Woking (so avoiding the crucial diamond).

norbitonflyer · 3 Aug 2026

Deepgreen said:
If it's not every working, surely that negates the benefits of leaving the points at Woking set?

I think the fact tghey are offpeak services is niot relevant. More significant is that 1000 to 1700 is the hottest part of the day.

AM9 · 4 Aug 2026

Given that we are heading for these higher temperatures every summer, would there be a case for temporary 'sunshades' over the most vulnerable parts of the formations at operationally crotical junctions? The anchoring points could be permanent but screens could be attached seasonally.

SWR Portsmouth services diverted via Oxshott on 3/4 August

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