Tank v Tender Locos

Very interesting. If my interpretation of this is correct, an Bulleid West Country comes out as 5P7F. I guess the SR did some judicious modification against known performance there. It penalises wide fireboxes quite severely; a King Arthur actually squeaks into 6P by my reckoning, but I don't think anyone would question which of the two classes was the more powerful passenger loco.

I can't see how anything can be classified as 8P at all, given the 4480 lb/hr firing rate limit - by the calculations provided a superheated loco can only just make 7P. Maybe 8P was recognition of locos with the capability for feats of high power for as long as the fireman could stand it.

Firing rate limit: 4480 x 6.18/20 x 0.00335 = 4.64 tons
Actual Merchant Navy: 48.25 x 30.7 x 0.00335 = 4.96 tons

Staying vaguely on topic, the L&Y Dreadnought tanks are just below the 5P/6P boundary, limited by their boilers.

Oh, and the LSWR D15 I cited earlier as hard done by at 3P - yes, it edges into 4P with cylinder tractive effort the limiting factor.

 

Taking big tank engines, if I have calculated right, the Billinton L class 4-6-4T just about squeaks into class 5 (limited by grate area - 26.7 sq ft * 40 * 0.00335 = 3.58tons drawbar pull). BR rated the N15X rebuilds as class 4P.

Tom

 

Here is a resume of the big GWR tank engines
81XX 2-6-2 ttank engines of which 9 built over 31,000lbs tractive effort.
4200 2-8-0 tank engines 31,450lb Tractive effort 7F
5205 & 7200 2-8-0 & 2-8-2 tank engines 33,170Lbs tractive effort 8F

 

It was the five locos of the second 31xx that had 31170lb reactive effort, but they were still only 4MT.

 

An interesting question is whether 4200s that received new front ends were given 19" cylinders, which would presumably make them 8F in practice. The 4253 group state that their locomotive (new front end in 1947) has 19" cylinders. The usefulness of such distinctions may be mulled over by considering what the actual difference in diameter and thus nominal tractive effort would be between 18.5in cylinders on last rebore and new 19" cylinders. In his book Cook tells us about a Saint that had nominal 18.5" cylinders bored out to more than 19".

Lester is of course quite right. Presumably they stayed at 4MT because of a "lowest number of" calculation.
Steve's excellent and much appreciated post doesn't tell us how xMT was arrived at. Was it just that 4P and 4F (or whatever) tended to coincide and 4MT was a shortcut?

 

What's "MT"? Would be e.g. 2P 2F where I come from (And another letter for braking capacity, but that's a different story...)

Tom

 

Any chance of making that available in a format other than JPG, which makes the wording unclear when magnified?

 

I doubt Swindon spent much time over it and wanted to keep the extra lettering as brief as possible; as far as they were concerned they'd be power class D, E etc. anyway.

 

The chart seems to indicate that MT and P classifications were essentially arrived at in the same way, see the drawbar pull calculation being given for "Passenger and mixed traffic engines." Certainly, if you have a boiler limited passenger class it's likely to have an equal or higher freight classification.

However, if you follow the chart to the letter, nothing with a GWR No.2 or No.4 boiler gets more than 3P or 3MT anyway, so I doubt Swindon were following it too closely. Of course, they weren't the only ones; a Southern U is apparently 4P6F if you follow the chart, as opposed to 4P3F as actually classified.

 

Makes you wonder whether any practical use was ever made of these classifications. I'm sure that most shedmasters would know which types were best suited for each assignment without reference to the power class.

 

Its well established that the Western region continued to use the GWR classification. I'm sure that shedmasters knew very well what of their local fleet was best suited to which turns, but when they had a foreigner on their hands I imagine the classification was useful.

 

I wonder whether much weight was given to official power classification or even technical suitability. I'm sure it was more a question of making the best use of whatever was available.

The two lines which I was most familiar with in steam days were the ex GE Southend Victoria line and the former London, Tilbury & Southend Railway. Both ran from the City of London to Southend, or a little beyond in the LT&SR's case, carrying similar traffic through similar country; yet the LT&S was an almost exclusively tank engine line, whilst the Victoria line was worked almost exclusively by tender locos. I suspect that this was due to historical circumstances; when the LT&S became independent of the GER it adopted a type of 4-4-2Ts which, with some upgrading, served very well until the 1930s, when the LMS introduced, as part of a modernisation scheme, the 3-cylinder "2500" class 2-6-4Ts, built specifically for the line. These were augmented after WW2 by some Fairbairn and BR Standard 2-6-4s, which worked passenger services until electrification in 1962.

Things were quite different on the ex GE line. Stratford M.P. district were never short of former express engines which were cascaded down to run the fairly tightly timed Southend services in hot competition with the rival system. Thus in my recollection almost all Liverpool St.- Southend Vic. services were hauled by a random mixture of B12, B17, B2 and B1 4-6-0s. L1 2-6-4s appeared from time to time but not regularly, they seemed to be mainly restricted to the shorter suburban routes.

 

Was there some sort of adhesive factor applied too?

 

Was motive power diagraming not centralised as a 'head office' function then? I didn't think it was done locally at shed level.

 

Diagramming, yes, they would also determine the power classification needed for the various duties and this would form the basis of the depot's allocation. However, a class 4 was a class 4 and, as long as the depot had sufficient of that power classification, it didn't matter which type of loco was allocated. That was the whole idea behind the power classification system. It was up to the depot managers to decide how they used that allocation.

 

Funny, at work now it is laid down by the train planners which type you would use. I'm talking EMUs admittedly, but much more central control!

 

Is that possibly due to seating capacity and, perhaps, maximum speed?

 

Capacity generally, being a commuter TOC I don't think seats are always the driver! speed too yes, plus with maintenance concentrated in fewer places and lots of siding berthing, the base plan cycles the units in every couple of days. The fleet team allocates units to diagrams but usually the booked type unless forced otherwise.

 

I don't think the ex LNER lines took the slightest notice of the LMS (BR) power classification. I don't recall it being mentioned in any correspondence.

 

Perhaps the BR power classification system was yet another case of someone in head office admin justifying their job. It's not as though the guys with the responsibility of actually allocating the motive power would be greenhorns.