Post by Anna Noyd-Dryver
You mean the failure to apply for a derogation?
I never got round to posting my observations on the other thread, so I'll
do it here.
The changes to the permitted clearances have been detailed in posts in the
other thread, with links to Roger Ford's writings on the topic. Safety
rules generally do change over time, and existing installations are
generally permitted to continue to operate. In this instance IIRC Network
Rail could have applied for a derogation (on a structure-by-structure
basis) but apparently chose not to (but not entirely - some structures
definitely have much tighter clearances than others).
There are three critical dimensions to be considered during this debate,
however. The first is contact wire to train roof (strictly, to the track).
There is of course a minimum for this, but over level crossings that
dimension is much higher. The dimension which Roger seems focussed on is
the catenary wire to bridge clearance. Between these two is the separation
of contact wire from catenary wire. Under bridges this is squeezed much
tighter - under very tight bridges the two wires are together (known as
'contenary', apparently). However on the GWML this happens comparatively
rarely - ie bridges are being raised by *more* than the minimum that they
need to. Presumably this is related to the desire to spec the GWML catenary
for 140mph - and presumably on the basis that if you're replacing a bridge
structure to raise it by X amount, the cost to raise it by another Y amount
is (presumably) comparatively small.
A tangled tale. I wonder if the figures used for vehicle
clearance have changed, and how they could actually be
Once upon a time... the Class 357 was the base design for the
whole Electrostar family. Strange as it may seem, the range of
wire heights with which it was to be compatible was very poorly
defined. Indeed, at that time, understanding and specifying its
infrastructure was something the railway was very bad at, whilst
somehow magically still wanting everything to be able to run
Work progressed, based on our interpretation of available data,
until a fateful meeting where someone asked "What about
When we looked at the new numbers we had been given, it was clear
that, worst case, the wire got alarmingly close to the vehicle
roof, and the downward curving horns at the ends of the
pantograph head were in danger of flashing over to the sides of
the pantograph well. (2).
The mod to the pantograph well consisted of removing a little
aluminium locally from the top of side wall, and adding a section
of insulating capping to ensure that the worst case clearance
distance to metalwork was still sufficient.
IIRC, the suspension was adjusted, to drop the whole train 9 mm.
It was also necessary to move some radio aerials away from the
vehicle centreline to position them a little lower on the roof
I simply can't remember now exactly how the mod was rolled out,
but would not have expected them to spend money making the
changes on vehicles to which the pantographs were not fitted.
Likelihood of actually running on Thameslink may also have been a
(1) Other gauging issues included platform edge/ step plate.
(2) This was much narrower, and had significantly higher sides
than any previous design. I had to fight for every millimetre
increase from the interference fit I inherited from the tender
outline design. A tedious time alongside a draughtsman with a
barely functional 3D graphics package was the only way we could
optimise the clearances between the various bits of HV roof
equipment, which from memory were just over 200 mm.
Chris J Dixon Nottingham UK
Plant amazing Acers.