Brunel's Atmospheric South Devon Railway - Common Misconceptions

This page is an attempt to bust some common misconceptions about Brunel's atmospheric traction system for the South Devon railway and why it failed.

It was Brunel's invention - WRONG

One, which may be a misconception or may be just laziness, is that it wasn't actually Brunel's atmospheric traction system at all. It was Joseph Samuda's. He invented it, and he had the contractual responsibility to maintain the installation, but he didn't have the resources to actually sort it when it became clear that the whole length was fucked. He had done a couple of others previously - one at Dalkey in Ireland, which is where Brunel got turned on to the idea, and one at Croydon, which was basically a demo system, and had all the same problems as the South Devon one, only the South Devon problems got all the attention because that installation was being used in anger rather than just as a demo. The Dalkey system actually pretty much worked, but there were under two miles of it and it was Irish (see later), so it wasn't really comparable.

Rats ate the leather valve - WRONG

This is probably the most common misconception and it gets repeated everywhere, including on sites that ought to know better.

The usual assertion is that the tallow mixture used to condition the leather caused rats to consider it a tasty snack and they promptly munched the lot. It's bollocks. No doubt the odd rat did have a cheeky nibble; sometimes they got sucked into the traction pipe. But it wasn't a significant problem. And if it had been, it would have been a doddle to solve it - simply add arsenic to the tallow mixture, in accordance with the general Victorian love of putting arsenic in bleeding well anything on the slightest excuse. (All the things considered in Feet of Clay as possible sources of the arsenic that was poisoning the Patrician are things that people did get arsenic poisoning from in real life because the Victorians put arsenic in them. Even the really weird ones that the Watch ruled out straight away.) Rats learn which food sources are dodgy by waiting for more adventurous rats to try it first and seeing if they get ill or not, so a few dead rats equates to a lot more rats deciding to leave it well alone.

The REAL problems with the leather valve

The above is not to say that the leather valve was problem-free. On the contrary, it was a pain in the arse, and the need to replace the whole bloody lot and the hassle involved in doing so constituted the major excuse for taking the final step and junking the whole system.

The basic problem was that leather as a material just wasn't up to the job. They used it because it was the best available material at the time, but that "best" wasn't good enough. Why Not Use Vulcanised Rubber, you may ask. Answer, because they didn't have it. Its development period pretty much coincided with the period the atmospheric system was in use on the line, and when they were putting it together there was no such thing as vulcanised rubber apart from experimental laboratory samples which were shit (burnt on one side and gooey on the other, with just enough good stuff between the two to show that it would be useful if you could figure out how to make it all go like that). The abovementioned valve replacement plan was going to use vulcanised rubber, which by then had reached a useful state of development, but it was too late and nobody (including Brunel) could be arsed with it any more.

In hot weather, tallow notwithstanding, the valve would dry out, become inflexible, and crack along the line of flexion where it was clamped to the iron pipe. In cold weather, it would soak up water, freeze, and again become inflexible, with the same result. The cracking would then develop into longitudinal tears, which made it leak like a bastard and ruined it. Such conditions also of course caused the immediate and catastrophic failure of the whole thing just freezing solid. Chemical reactions between tannins in the leather and iron oxide from rust caused the leather to deteriorate more rapidly, and being repeatedly doused in sea water didn't do it much good either.

The Dalkey system had, in addition to the leather flap valve, a hinged iron flap over the top of it to act as a weather shield. It wasn't completely effective but it did make a major difference. The South Devon system, on the other hand, was built without this iron flap, partly because of the huge inertial forces to be contended with in flipping it out of the way at the high speeds envisaged, and partly to cheap out. The Croydon system didn't have it either, presumably because it was only a demo and they didn't think it was worth the extra effort. Accordingly, both the South Devon and the Croydon systems suffered the same weather hassles, only the South Devon one had it worse because the Croydon one didn't have to cope with being inundated with sea water in the winter.

The other major weather problem concerned the method by which the valve achieved a seal. The idea was that the tallow mixture would be a solid under normal conditions, but when heated a few degrees (by radiant heat from a charcoal burner underneath the train) it would melt and release the valve. Then once the train had passed it would solidify again and glue the valve down in a nice unleaky manner.

Quite a neat idea, and indeed still in the ideas library of modern technology - for use under appropriately controlled conditions. The problem, of course, was that this application involved conditions that weren't controlled at all. So in hot weather the mixture would be liquid all the time, and in cold weather it would be solid all the time. To make it work, it would have been necessary to use a composition with a melting point higher than the maximum hot-weather resting temperature, and maintain the sealing surface a few degrees below that temperature at all times by means of a thermostatically-controlled heating system. That would be easy to do these days, with an electrical heating system, but it would still use a lot more energy than the actual traction requirement. It would be extremely difficult with 1840s technology, and the fuel consumption would have been appalling.

It probably never worked properly even when the weather was neither too hot nor too cold, either. The radiant heater would have had to be fucking fierce to heat the goo enough to melt it when the train was at speed, and if it was that fierce then when the train stopped in a station it wouldn't just be melting the goo, it would be setting the bloody stuff on fire. Since that, at least, did not happen, I don't believe the melting did either, and it probably just ended up relying on the goo not going completely solid anyway. (In fact I'm not completely sure if they even bothered with it at all on the South Devon. The Dalkey system did have it, but it didn't work.)

The line's notorious gradient profile is the result of it being designed around the atmospheric system's hill-climbing ability - WRONG

It's the other way round. The route was decided, the plans submitted to Parliament, and the Act obtained, BEFORE anyone had raised the idea of using atmospheric traction. The Act got the Royal Assent in July 1844; the plans at that time were, per Brunel, for a double-track railway using conventional traction. It wasn't until August that the Board of the company began to discuss the possibility of using atmospheric traction, and the decision to actually do it was made in September. The reason for considering it in the first place was that the route, with its heavy gradients, was already decided on, but Brunel got worried about the ability of locomotives to cope with those gradients and came up with the idea of using atmospheric traction to overcome a problem which was already known to exist.

When the decision to use atmospheric traction was taken, the decision to build a single line instead of a double line was made along with it. A single line with atmospheric equipment fitted worked out cheaper than a double line without it, and it was thought (without anything noticeable in the way of proper justification) that the atmospheric system would improve working so much that the capacity of the line would still be adequate. Given the amount of fucking about required to deal with the difficulties caused by the impossibility of running the traction pipe through pointwork, I'm buggered if I can see why they thought this, but they did.

Quite why the route was laid out with such horrendous gradients in the first place is something of a mystery. To be sure, there simply isn't a way between Exeter and Plymouth, on any possible route, without encountering at least comparably difficult terrain, but it still doesn't need to be as bad as it is. Cheaping out, I suppose, though the existence of the sea-wall section tends to act against this hypothesis. Dainton, in particular, could have been avoided altogether with a sufficiently long tunnel - of a length for which precedent existed, in the form of Woodhead rail tunnel (opened 1845) and various canal tunnels such as Standedge which were considerably older. Even without such major works, the topography still permits an easier route for that same section of the line than the one that was used, so quite what they were on is not easy to discern. The decision process seems to have been something along the lines of "it's going to be shit in one way or another no matter what, so fuck it, let's just go straight up and over", which is a bit daft, especially since they were still planning to use locomotives at that stage, but there we go.

I suppose it is vaguely possible that Brunel had it in mind to use cable haulage; I have seen the suggestion made, in the context that he (may have) had it in mind in case locomotives turned out not to be able to cope with the 1 in 100 up through Box tunnel. But that bank is dead straight, as are other major quondam cable-worked inclines like Cowlairs and Camden, whereas the South Devon spiky bit is really quite wiggly. Being wiggly doesn't positively prevent cable haulage, though it does make it rather more of a pain in the arse; but the route still conveys no impression of having been designed with cable haulage at all in mind, and I don't think the idea really gets us anywhere usefully nearer an answer.

Atmospheric traction makes light of gradients - WRONG

This is perhaps an excusable misconception since the Victorians suffered from it too, but again, it is hard to see why, because it only takes a small amount of mental arithmetic to show that it is bollocks. If the system ever had got beyond Newton Abbot, though, it would have hit them in the face. They were running trains of up to 120 tons along the level stretch where they did install the atmospheric system. If they had tried to take those over Dainton they would quite simply not have made it: they would have ground to a halt, and only dividing them into several smaller trains would have got them going again.

No bugger ever mentions this. Everything I've seen written about it, whether Victorian or modern, concentrates exclusively on the positive aspect of atmospheric traction on hills: the independence of friction between wheel and rail. (Note: railway people, stop calling this "adhesion" for fuck's sake, it's friction you tits. Adhesion is what glue does. Adhesive forces can contribute to friction, in greater or lesser degree depending on what materials are involved, but it certainly isn't the same thing, and you can still get friction when the adhesive component is zero.) An atmospheric train cannot "slip to a stand" with its driving wheels revolving to no effect (like this bugger did, with me on it).

Certainly that is an important advantage, and one which tended to be overstated at the time, because people had not yet properly got their heads round important concepts like coefficients of friction or the weight distribution of locomotives, and tended to severely underestimate the potential of conventional friction drive. But they did have their heads around the concept of pressure, and the overstatement of the positive aspect is no excuse for the universal tendency, now or then, to ignore the negative aspect, both because it fucks the whole thing irretrievably and because it is so trivial to figure out that that is the case.

Independence of friction does not mean that atmospheric traction has unlimited hill climbing ability - far from it. The tractive force available for a locomotive-hauled train can be increased effectively without limit (or at least until the couplings snap), simply by adding more locomotives. But the tractive force available to an atmospheric system is subject to a hard limit set by the diameter of the traction pipe and the value of atmospheric pressure. Once you hit this limit, there is fuck all you can do about it. (Unless you rip the whole lot up and start again, and even then you can't get near matching even one locomotive.)

Calculating the absolute maximum tractive force under ideal conditions is a piece of piss, since it's just πr2 x atmospheric pressure. The plan was to use a 22-inch diameter traction pipe over the hilly sections, if they had ever got that far. This gives a theoretical maximum tractive force of around 2.5 tons-force. (To give some sort of point of comparison, even if it is next to useless, the GWR Iron Duke class locomotives, introduced in 1847, had a rated tractive effort of 3.6 tons-force; the GWR's ultimate extension of the same technology, the King class, got up to 17.8 tons-force.) Dividing by the maximum gradient - in this case, 1 in 36 - gives the theoretical maximum train weight over Dainton as 90 tons. And that is all you can ever get.

In practice, of course, conditions are not ideal and you don't get anything like that. For a start you don't get a perfect vacuum ahead of the piston. Exactly how well they did on the South Devon I don't know, but the Croydon system is reported to have achieved 25 inches of vacuum in the pipe when it was new. If the South Devon could have managed that, it gives a theoretical maximum train weight over Dainton of 70 tons. They more than likely didn't, what with all the leakage; if they managed 20 inches, that brings the maximum down to 56 tons - ie. less than half what they were running along the flat. And this is of course ignoring friction and other losses, so that's 56 tons moving imperceptibly slowly. If we assume a quarter of a ton-force of drag, which is probably reasonable, we're down to 47 tons; leaving some operational margin, 40 tons, and we're still idealising things so that's still an optimistic figure.

In short, if they had got as far as trying atmospheric traction over the banks, far from making light of them, it would have been really really shit.

So why did it fail, then?

Well, we've already covered the most immediate cause - failure of the valve due to tearing of the leather, and deficiency of the sealing system, both resulting from weather conditions (not bloody rats). But that is not all there is to it.

Brunel cocked up the calculations for sizing the pumping engines. Brunel was a very successful engineer in civil and structural matters, but he was shit at anything mechanical or dynamic. (Examples: his locomotives were shit, his propeller for the Great Britain was shit, his arrangements for launching the Great Eastern were shit, his piled track was shit.) I can get similar figures for the required pumping engine power by doing very simplistic calculations in my head from the only readily accessible "results" figure for the system output (force x distance along the track) - and I know they are wrong, because they ignore everything I don't have figures for; all the stuff like leakage, pump inefficiency, engine manufacturers quoting higher output figures than you actually got, and every other source of inefficiency, inaccuracy and loss.

So the engines installed were undersized, and could not cope with the task they were called on to perform - unless they were thrashed. Of course, one of the great things about a steam engine is how well you can thrash it. But doing so absolutely canes the thermal efficiency, which is ropy enough already. So it is not something you want to be doing as a matter of course. Thrashing a steam locomotive to overcome a heavy gradient is OK, ish, unless you're the fireman, because it's a temporary expedient, but using a stationary steam engine in thrash mode all the time just makes it use stupid amounts of coal, and this is exactly what happened.

And of course when they weren't pumping they still had to burn coal to keep them ready for action. Since the proportion of idle time was vastly greater than for a locomotive, that too resulted in disproportionate fuel consumption.

They never sorted out the telegraph. There should have been a telegraph installed in all the pumping houses, so they could let them know when a train was due and they needed to start pumping. But they fucked around and pissed about and never got their fucking act together to install the bloody thing. So the pumping houses just had to start pumping whenever the timetable said a train was due, and then carry on pumping until it actually turned up. This of course led to even more waste of fuel when the trains weren't running to time.

A significant part of the justification for installing the atmospheric system in the first place was that it was expected to be cheaper to run than locomotives. Quite why they had any confidence in this, given that there was plenty of data to estimate the running costs of locomotives but fuck all for atmospheric railways so the figure was basically guesswork and wishful thinking, is obscure, but that's what they did expect. In the event, with all the thrashing and idling and redundant pumping, it ended up costing about three times more than locomotives to run. (How it would have worked out if everything had worked as intended is again guesswork, but I strongly suspect it would still have been a lot worse than locomotives; if nothing else, a locomotive connects the steam engine directly to the wheels, whereas atmospheric traction interposes a lossy and inefficient method of power transfer. It would take a lot of efficiency advantage of stationary engines to make up for that - and at that period stationary engines hadn't started taking advantage of their stationariness to incorporate efficiency measures which aren't practical on a locomotive (apart from condensers), so unlike the situation obtaining today, where stationary steam plants are the second most efficient (behind ship-sized two-stroke diesels) heat engine in use, they were even worse than a locomotive. The boiler pressures, for a start, were ridiculously low, whereas locomotives at least were trying to get them up, even if that was for reasons of power-to-weight rather than efficiency at that time.

So what with the grossly excessive running costs and the expense and disruption of replacing the entire length of the flap valve, they ended up deciding that it was far more trouble than it was worth and got rid of it.

Anything other than plain running was a pain in the arse. The atmospheric system can't do two very important things: it can't go through pointwork, and it can't go backwards. So while it's fine for tanking along plain track hand on cock and mind in China, it can't do any kind of shunting or marshalling operations, and it can't put a train in a loop part way along a single line section (which it all was) to let another one come the other way. For shunting, they had an auxiliary traction pipe in stations that ran alongside the track with a piston that you could tie on to the train with a bit of string. That let you pull one way, but not, I think, the other; or at least not in any remotely practical way. Regardless, there still was plenty of "other", and for that it was either use a horse or shove it along by hand. (Either of which, I rather suspect, would probably have been less fucking around most of the time than using the pipe. Shunting horses, after all, remained common long into the 20th century.)

For two-way working, they were just plain fucked. They had to run trains through in one direction, then reverse the whole system and run trains through in the other direction. There was just no way to send trains through in both directions at the same time and stick one in a loop to let the one coming the other way go by.

Perhaps surprisingly, they did manage to sort out a way of handling level crossings. Something or other went up and down to bring the road surface and the top of the traction pipe to the same level when the crossing was set for road traffic. I'm not sure quite what, but I'm pretty certain it was some kind of movable ramps in the road rather than having the pipe itself go up and down, because making that seal properly would have been an utter cunt.

What about Dalkey?

The Dalkey installation in Ireland was not the abject failure that the South Devon was. It ran for 11 years, and was torn up not because it didn't work, but in order that they could convert the track to standard Irish gauge and connect it to the rest of the rail system.

So what was different? Well, for one thing, it was much smaller. It was less than two miles end to end, and didn't stop on the way. So it was vastly less of a burden to operate, with only one pumping engine, and not a stunted one either. And it had the weather protection flap over the valve so deterioration of the leather was much reduced.

And they were Irish, and didn't care about it having shit bits. The traction pipe did not go all the way. It stopped about 500m from the end. Trains arriving at that end would coast from the end of the pipe to the end of the line. Trains departing had to have some poor sod push them by hand until they got to the pipe. It hardly needs saying that this was epically shit, but they never did anything about it. Making sure the flap valve re-sealed properly after the train passed was achieved by having some other poor sod walk along the line pushing it down, and they never implemented a less shit alternative to that either. Irish railways were well into doing things like this. So were English ones, especially in the beginning, but they almost invariably soon started fretting over it being shit and replaced it with something not shit as soon as they could get it together, whereas the Irish ones just didn't give a shit and left it like that for ever. I suppose if you have a large enough supply of poor sods you can claim anything works no matter how fucked it is.




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