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Anyone who travelled between reading and Paddington today will know how badly the services were affected by (I believe) signal problems but I believe this service suffered additionally from mechanical problems as well

I joined the delayed service at reading to get to Thatcham but it seemed to suffer from severing jolting - almost as if the two power were fighting each other or out of sync. Is this possible? At one point I wouldn't have been surprised if it was taken out of service before reaching its final destination
 

The service was 40 minutes late leaving Paddington while the man with the spanners attended to a fault.  It is possible it was only partly rectified.

I was on a service from Plymouth last week which was running on only one power car and that had the symptoms you describe as it approached Reading.  The train manager said on the PA it was a by-product of only running on one power car that it juddered at low speed.

Thanks Bobm but if it runs on one power car isn't that just like having a desiel powered set or is there more to it than that ?

I'm no expert but the Train Manager said the driver would experience problems when running at low speed.  I certainly noticed some juddering as we approached Reading West and slowed for the inevitable wait for a platform at Reading itself.

Are you sure this wasn't just severe wheelslip caused by low adhesion (leaves on the line)? Today seems to have been the worst day so far, and our climb from Chippenham up to Box Tunnel this evening was one of the worst I've experienced in 5 autumns of commuting. I could quite easily have described it in the same way as you described your journey, if I hadn't known what it was. That was on 2 power cars - don't think we would have made it on one.

Actually it was - service was terminated at Westbury due to late running!   ;D

As John said, most likely wheelslip.  Both power cars have an independent WSP detection system, so if the rear car is slipping under power the engine will automatically cut power momentarily whilst the front power car might be fine, and vice-versa.  Under braking you can also have a situation where some axles of the train are slipping whereas others aren't.  Both situations often result in a juddering sensation.

If the rear power car fails, then you are left with a close equivalent of a standard loco hauled train with the engine at the front. I would not expect juddering in such conditions.

If however the front power car fails then I would expect some deterioation in ride qaulity. With the engine pushing under steady conditions, all the couplings will be compressed. If the driver then coasts or brakes the couplings would be stretched.
Frequent changes in gradient, power setting, or braking, could result in couplings alternatly slackening and being pulled tight, a poor ride resulting.

Some trains used to operate regulary with the engine at the back, and IME the ride was then inferior to that experienced with engine at the front.

There isn't much in the way of slack in the couplings between vehicles on an HST set to make a difference if it is running one engine only.

The only time you would experience noticeable juddering is through wheelslip or the power being knocked off to quickly.  On both scenarios juddering would be more noticeable at low speeds.

 

Thanks everyone for your comments and the judder was more noticeable when pulling away from a stop.

There were two HST FGW drivers tweeting yesterday saying it was the worst day for adhesion in deveral years & that 30mph was the fastest they wanted to travel at.

Indeed all the East Coast (and Anglia intercity) electric services are push-pull.

Does sound like wheelslip as well.

I was on an HST a couple of days ago from Bath Spa - Bristol Temple Meads.  On departing Bath the train jolted and juddered pretty much all the way to Oldfield Park. I did have a feeling at the time something wasn't right as we appeared to be slowing down again. But we picked up speed and proceeded to Bristol with only slight delays.

That being said I was also on the 21:31 Weston-Super-Mare to Bristol Temple Meads train a few weeks back, also a booked HST. I know the driver of that service. We left WSM bang on time and when we go to Bristol we were around 10 minutes late because of low adhesion.

Of course I and I'd say most of us on this forum know the reasons for low adhesion. I said to the driver I generally don't mind being late especially if it means I get there in one piece. They said they wished all passengers had the same thought process.

I'd say nearly every train I've used this ppast3 - 4 weeks has some form of delay. Whilst at times it may be frustrating, there is not an awful lot that can be done so the Drivers and Staff have my sympathies

TT :-)

I agree with all you said in this post TT (especially about the recent run of delays) but I quoted this bit because last night tempers were  running high at Reading and the  staff really didn't deserve the rudeness beng thrown at them

I did find myself wondering last night if I had to choose between a shiny new station or reliable signalling between Paddington and Reading what would it be...

I was on the Golden Hind (1803 ex-Paddington) yesterday (21 November) and we only just made it up Rattery Bank after leaving Totnes (when I got home I felt like I had been shaken to bits :D).  This has always been a difficult climb at the best of times but yesterday was the worse I have ever encountered in nearly 30 years of train travel to the far West.

There is a difference between wheel slip and wheel slide

Wheel slip is where there the tractive effort exceeds the wheel to rail friction - prevention is by monitoring the current in the traction motors and comparing the currents and reducing the power to a wheel set that is about to slip, excessive slip causes burn marks on the rail head and flat tyres the wheel set will need to go the wheel lathe; can also on older type wheel sets lead to the tyre moving on the wheel, used to be marks on the "Gibson ring" to help spot this.

Wheel slide is where the braking force on the wheel causes it lock because of the loss of friction between the wheel and rail. Prevention is by measuring the rotational speed of each wheel set, the best solution is to compare wheel set to wheel set, if a slide is detected to release the brake monetarily excessive slides cases flat tyres, if you ever travel in a carriage and hear a tap tap tap and the train move off which get faster in rate as the train picks up speed  that's a flat tyre and the wheel set will need to go the wheel lathe.

The visit to the wheel lathe is an expensive business

Which probably explains why FGW appear to have so many Mk3's in service with wheel flats. Hardly a week will pass by where I don't get to ride in at least one Mk3 with a flat. Not too much of an issue, noise or comfort wise, in a day coach, but a right PITA if you're over a bogie with a wheel flat in a sleeper coach.

Even if it was a cheap exercise, i'm sure endless short-formed HST sets running around to sort out flats wouldn't be too welcome either - especially with the current railhead conditions being so bad.

The track engineer is not keen on flat tyres it undoes his / her tamping

Thanks for pointing that out, Electric train: it's not an aspect I'd actually thought of before.  :)

This mornings 0830 from Truro to Paddington had a large amount of wheel slip/ spin pulling away from every station (to Plymouth at least where I left the service) Cornwall had a heavy frost last night so I am assuming this would be the contributory factor. The roads were untreated when I dropped my daughters to their nanny's at 0700 this morning, and there was several patches of sheet ice I noticed between Redruth and Falmouth along the A393 around Lanner.

Wheel flat on my 0626 Clifton Down to Bristol Temple Meads Class 150/1 yesterday morning. Very noticeable at low speed and had me thinking the sound is very similar to what one used to get when travelling over jointed rail.