New-build steam strategy coordination?

Ah, as I understand it, the wheel pattern is different (number of spokes), but that's about it, isn't it? (Was that more 'Swindon' influence?). Wheel diameters, wheelbase, cylinders etc were the same, which was why the idea for the move to a 3MT was there.

 

I think that it was after an incident, possibly with a fatality, involving trains passing at a set of water troughs. Water was overflowing from the tender of one locomotive; this hit the (angled) cab front of the locomotive of the other train pretty much square on, & the front cab window failed with the impact. The thinking was that by having a flat front, there couldn't be a repeat occurrence.

 

This was actually tested on the LNER after the accident, and the type of glass used on the A4 was changed as a result of the tests made. The flat fronted cab withstood the pressure of water better than the angled one as the pressure was even across the whole of the glass. The A4 window shattered as the water pressure was concentrated in one area. Thompson lamented the death of the inspector who was stood behind the glass on the run, and made the decision for flat fronted cabs on this basis.

With the exceptions of the A2/1 and A2/2 - whose cabs were based on the classes designed/rebuilt from, Thompson's Pacifics were all intended to have flat cab fronts as a result and this was evident with the A1/1 and A2/3 that emerged. You will note that Peppercorn's cab, although angled as per the A4s, actually has differently shaped cab spectacles which was an alternative and much improved design that offered better vision and also better prevention of such an accident.

What is disappointing to me is that Thompson's safety conscious point was interpreted as "retrograde" by several historians without thought for the decisions made behind it. It was for safety reasons. I accept the advantages of the angled cab glass on the A4, but almost every steam locomotive ever built in this country with cab spectacles had flat fronted cabs, so the supposed advantage of the A4 wasn't shared across the majority of other steam locomotives. Was the lack of reflection in the glass really such a big deal if safety was the foremost concern? But that is for the Thompson thread...

I'm not being serious when I say "build a Thompson Pacific". There's no real business case for one even if it would be technically easier to do. But I would like one all the same. I think many people would enjoy seeing one of each design line up in BR dark green one day, as the originals did apparently on some occasions.

 

There might have been a small aerodynamic advantage in the A4 having the angled cab. Bear in mind the whole idea of the loco was to streamline it for high speed performance.

 

For a start, the Class 3 axleboxes are smaller in width, consequentially the cut-outs in the frames for the horns/axlebox guides are smaller in width. The axles on the Class 3 are 1/2" dia smaller on all three diameters. As a result of this the wheel bosses are smaller in size on the Class 3s. The frame plates are 3/4" longer on the Class 3s. Different design of hind dragbox. The Class 3 has a separate firebox support stretcher. Shall I go on?

 

That absolutely was the original reason for the A4 cab being angled - that wasn't in dispute. It was a nice advantage to the design that it didn't reflect light glare at night or during the daytime, but as said above the unfortunate consequences of the design was the loss of an inspector's life. I don't believe anyone could have predicted that mind, so there's no blame - it's just a sad unfortunate occurrence.

 

Bringing this back to Bulleids, it was also a later design modification on those engines (entirely independent of the main rebuilding) to go from flat-fronted to wedge-shaped cabs, again for reasons of better visibility. Of course, there was no issue with water troughs on the Southern.

Tom

 

That explains the different wheel patterns. Thanks for the more detailed info; what a pity, then, that there are references in various places that say that they were the same...

Hey ho!

 

Fairly soon there will be few of us alive who remember real working steam in any form so I hope this argument doesn't hold up for very long or there will be no more new builds. Many of the infamous 'facebook new builds' seem to be led by young people without the necessary skills or experience but it is interesting that these apparently hopeless schemes often involve pre-grouping locos. Even if they fail now let's hope some of the participants will succeed in building their dream locos some day in the future when they're older, fatter, wealthier and (optionally) wiser.

 

Well, I at 24 have little interest in the railways of my lifetime, and not much even in BR standard steam, although I'd be interested in a sort of super-71000 four-cylinder 2-8-2 or 4-6-4... my interests are mainly Bulleids and late-Victorian/Edwardian types, with a bit of a soft spot for some of Urie and Maunsell's interwar designs.

 

Going back to this, it depends what you needed. At the time the LMS still had the hangover problems from Fowler and the board adopting the Midland's small engine policy. During wartime, passengers still needed to be hauled around the country, and although they could reduce speeds/make trains longer up to a point, the LMS still lacked enough big class 7 (LMS Class 6P) and class 8 (LMS class 7P) engines and were having to double-head trains.

AFAIK many of the wartime Duchesses were half-built when war broke out, which was enough to delay their completion, but scrapping a part-complete engine when it could be completed wasn't the wisest move. There were also clearly advantages in completing engines to a standard design rather than going off on unproven flights of fancy like newly designed Mikados. Ultimately the LMS's wartime policy was to churn out the standard 8F and black five designs en masse. The wartime duchesses represented a small proportion of the total construction.

The rebuilt scots were also a proven design (as 6170 was the prototype). Their boilers needed replacing and they probably would have been replaced earlier were it not for the war.

Also, remember (from a point earlier) the Rebuilt Jubilees added two spare boilers to the Jubilee boiler pool. The 5-year old obilers wouldn't have been scrapped, but reused on existing engines. Because it takes time to repair/refurbish a boiler than it does for the chassis, you have more boilers than chasses so that when the chassis has been refurbished a new boiler cna be mated with it.