Tay bridge restrictions?

[marks87]

I mentioned in the EC First Class menu thread that today's 1S11 10.00 KGX-DEE was terminated at Edinburgh because one of the power cars had failed.

When the guard came round to make sure everyone knew what was happening with replacement buses etc., she said that the reason for terminating was because they wouldn't be allowed on the Tay Bridge with only one power car working.

Is that true? And if so, do we know exactly why such a restriction exists?

 

[deltic1989]

Not entirely sure if this is correct, or even close to the reason. But, could it be that such a restriction exists in the event that the other power car fails and you are left stranded in the middle of the bridge waiting for a T-Bird?

 

[Nym]

Could be, but would that restriction then not extend to any MU on it's 'last engine' or consequently onto anything loco hauled?

 

[Eagle]

Wouldn't that apply to the Forth Bridge as well (although it is shorter)?

 

[philjo]

Maybe because the Tay Bridge is single track so anything breaking down would block the route in both directions - whereas the Forth Bridge is double track?

I would have thought it more likely that the train was terminated in Edinburgh so it could go into the depot for attention.

 

[khib70]

Maybe because the Tay Bridge is single track so anything breaking down would block the route in both directions - whereas the Forth Bridge is double track?

I would have thought it more likely that the train was terminated in Edinburgh so it could go into the depot for attention.

Unless my memory is playing tricks, the Tay Bridge is in fact double track.

Haven't heard of any restrictions on HST's of the type mentioned in the OP, though.

 

[matchmaker]

AFAIK, the only restriction on HST's in Scotland with only one power car is that one isn't allowed north of Blair Atholl unassisted.

 

[marks87]

I would have thought it more likely that the train was terminated in Edinburgh so it could go into the depot for attention.

I think you're right. Although with it terminating at Dundee anyway, I suspect it was due to end up at Craigentinny regardless, running ECS back through Fife.

Unless my memory is playing tricks, the Tay Bridge is in fact double track.

Yep, it's double track. Bouch's original bridge was single-track, but Barlow's replacement was double, which is what stands to this day (he says poetically...)

AFAIK, the only restriction on HST's in Scotland with only one power car is that one isn't allowed north of Blair Atholl unassisted.

Could it be that the guard was half-correct, in that the restriction does apply but only S/B, because of the gradient to get onto the bridge (assuming that it's gradient issues that require 2 cars north of Blair Atholl).

Out of curiosity, what is the maximum speed of an HST with only one power car? And are there other issues?

 

[Zoe]

Unless my memory is playing tricks, the Tay Bridge is in fact double track.

It is double track but unless there have been some recent changes, only one train is allowed through the high girders at a time.

 

[matchmaker]

Could it be that the guard was half-correct, in that the restriction does apply but only N/B, because of the gradient to get onto the bridge (assuming that it's gradient issues that require 2 cars north of Blair Atholl).

I don't think that the climb on to the bridge compares to the climb up to Drumochter!

 

[The North Briton]

I would have thought it more likely that the train was terminated in Edinburgh so it could go into the depot for attention.

I think you have hit the nail on the head. HSTs can (and have) run through to Aberdeen on one power car, the only restrictions are missing out the stops at Inverkeithing and Stonehaven (sorry, can't remember in which direction) because of the gradients.
As the train was only going to Dundee, it wouldn't have the option of going to Aberdeen Clayhills depot for attention, so it was probably a wise decision to get it onto Craigentinny sooner rather than later.

 

[Liam]

Unless my memory is playing tricks, the Tay Bridge is in fact double track.

I'm sure I remember reading a few years go they were going to single due to uneven wear. Obviously this hasn't happened yet, if it ever does

 

[scotsman]

Only one train is allowed on the central span of the Tay Bridge at a time due to, IIRC, a weight restriction no doubt originating from the fact that the bridge was only built to handle one train at a time.

Trains on one power car cannot stop at Inverkeithing in the up (Edinburgh) direction due to the incline immediately after the station

 

[ChrisCooper]

The problem with HSTs on one powercar is that they don't have great tractive effort, being fairly lightweight and designed to work in pairs, so they can really struggle on hills, especially when starting. Other places where there are problems are the Lickey Incline and the Devon Banks. If a loco is available they can be hauled. A HST on one powercar doesn't have any speed restriction, as long as the other powercar is there (without they are limited to 100mph due to the fact the rear powercar brake valve is needed to stop from above 100mph) but oviously will have trouble reaching and maintaining 125mph on anything but downhill (had one on one powercar doing 125 down stoke bank a few years back, although it had been very slow at the summit). Also, when a HST looses one powercar it's power is more than halved, as normally only one powercar provides train supply so the the other has more power available for traction, wheras unless the fault is such that the dead powercar can be left running for train supply, the one remaining powercar has to haul/propel the whole train and provide train supply.

Another problem you can get with HSTs on one powercar is fuel consumption. The remaining powercar is going to be using much more fuel, afterall it will be spending far more time on full power. On long runs that can mean the other powercar runs out of fuel.

 

[robertclark125]

I remember a HST from Aberdeen to London, which was on 1 power car from Montrose, and it didn't have any assistance. I saw it leave Kirkcaldy over 40 minutes down, so it had to have gone over the tay Bridge on its own. So HSt's on one power car are allowed southbound.

And speaking of climbs, it's easy to forget the climb northbound from Dundee Dock Stret tunnel to Camperdown street level crossing, which is just as severe as the climb to the bridge. So, no difference for an Aberdeen HST north or south bound.

 

[KBD1]

It is double track but unless there have been some recent changes, only one train is allowed through the high girders at a time.

It's still only one train at a time over the high girder spans for now.
NWR are proposing to change it some time in the near future 2012/13.

Regarding HST with one power car, that could be a problem if it fails when crossing the high spans.

 

[Jonny]

It's still only one train at a time over the high girder spans for now.
NWR are proposing to change it some time in the near future 2012/13.

Regarding HST with one power car, that could be a problem if it fails when crossing the high spans.

Northbound, though, it could just coast off (there is a grade downhill towards Dundee on both "Tay Bridges" IIRC)

 

[marks87]

Northbound, though, it could just coast off (there is a grade downhill towards Dundee on both "Tay Bridges" IIRC)

I think, but I'm not 100% sure, that the rail bridge is flat through the high girders, then has a downhill grade towards both Dundee and Fife.

It would certainly make sense, because the original point of the high girders was to preserve the shipping lanes along the Tay.

 

[KBD1]

The Tay Rail Bridge is mainly on a level gradient, untill it runs down into Dundee, past the old station.
Very slight gradient coming off at Tay Bridge South signal box.

 

[ChrisCooper]

I seriously doubt it's got anything to do with the fact the other powercar might fail. If that was the case then loco hauled would also be banned unless double headed. The only concern with regards to reliability with the remaining powercar would be fuel state. That might be an issue depending on where the train was supposed to be fueled and how long it had been running on one powercar. If it was due to run to Dundee and then return to Craigentinny for fuel then there might have been concerns that it wouldn't have made it.