# why weren't tank engines more common?



## lownote (Jan 3, 2008)

It's probably a naive question. But why were tank engines mostly confined to yard work? Thinking about it, it would seem you could gain significant efficiencies with a tank engine, especially with a saddle tank, because the boiler would be preheating the water supply all the time. And you'd be adding some weight over the drivers, instead of behind them in a tender. And once you'd gotten up to steam the thermal mass of the saddle tank would act as insulation, reducing the costs of fuel. So even if you had to pull a tender, it seems as if there would be advantages to a tank engine.


I can see where you could not get a whole supply of water into a saddle tank, but it would seem like it might be a good way to provide feedwater heat. I assume that the biggest problem with saddle tanks is visibility for the engineer?

Am I right that tank engines are more common in the UK? And that it probably had something to do with the relatively shorter distance between towns?


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## dmk092 (Jan 5, 2008)

I dont know for sure but from what I can infer the major reasons against having tank road engines would be:

even if you gained a boost in adhesion, this would decrease over the course of the run, which could lead to problems


even if you did have a tender to supplement water supply, from what Ive seen in photos the boilers of US mainline engines tended to take up most of the loading gauge, so a tank would necessitate a smaller boiler, which probably wasnt worth it



also, while I havent crunched any of the numbers (I refuse to do any thermodynamics for a month since I just took my heat transfer final) I dont think it would result in that great a boost in efficiency. If the boiler is insulated, than your not really going to heat up the water significantly. If the boiler isnt well insulated, than your system isnt efficient anyway and you should probably go back to the drawing board. This is a pretty qualitative assessment, but if anyone wants to look up the rate of water consumption of an engine, boiler surface area, operating temps, heat transfer coefficients, etc and plug it into a TI 89, Ild be interested to know the answer.


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## Dwight Ennis (Jan 2, 2008)

I suspect it was mostly simple logistics. Without a tender, fuel quantity was limited and tank locos had to stay close to the fuel source and water tank. If you're going to haul a tender anyway to suplement fuel and water, why bother with a tank which, as CMK pointed out, may make the boiler smaller? 

Anyway that would be my guess.


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## Great Western (Jan 2, 2008)

Yes indeed. Tank locos were very widely used in both the UK and the rest of Europe. The shorter distances between stops and size considerations - trains running in already built up areas as opposed to virgin territory as often the case in the United States - generally dictated the choice of loco type.


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## David Buckingham (Jan 2, 2008)

One other thing to think of is the weight per axle on a lot of lines even express in the UK were limited to 22 tons or thereabouts per axle. 

A lot of American especialy narrow gauge were very lightly laid 

Dave


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## NTCGRR (Jan 2, 2008)

good question..


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## R.W. Marty (Jan 2, 2008)

Hi all,
Another thing to consider is that tank engines were designed to run in either direction without having to be turned.
The 0-whatever-0's were usually in yard or industrial service but something like an 2-6-2 was a road engine on many shortlines that didn't 
have or want to bother with or have room for tables, wyes, or ballons.
The pilot truck and trailing truck were virtually the same on many of these engines.

Look here http://narrowmind.railfan.net/BLW/BLW-262goodfor.jpg

Later
Rick Marty


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## lownote (Jan 3, 2008)

I'm guessing it might partly just be tradition.The history of industry and industrial production suggests that way more than you might think--and way later than you might think--whole industries just ran on custom: "this is the way the guy who taught me did it." They have the tooling for tenders, they have guys who are good at making tenders at a reasonable cost, they don't have a lot of saddle tank guys in the shop. Also in the US fuel was cheap compared to Europe, I think that's true even compared to England, which has a lot of coal. It was definitely true about wood.


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## Bills (Feb 20, 2008)

I believe that tank engines were very popular in urban setting such as NYC.


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## Dwight Ennis (Jan 2, 2008)

I'm guessing it might partly just be tradition.
I donno Mike... steam locomotives went through a constant evolution during their roughly 100 year history. Not only were they constantly being improved by their builder (Baldwin, Alco, etc.) but each railroad was also constantly experimenting with this and that. Even locos of the same basic class on the same road varied in the fittings and details as new things were tried to match the loco to the terrain the railroad ran through, the quality of fuel and water available, and the job it had to do. 

Examples: 

[*]The SP cab forwards were a direct result of the loco having to pull heavy trains through many tunnels and showsheds in the Sierras, asphyxiating the crew. [*]Employing anthracite waste as cheap fuel led to the Wootten firebox and Reading's Camelbacks. [*]The lack of a ready source of coal in the west led to oil firing ()eems to me hard vs. soft water in different areas also led to variations, but I can't remember what they were offhand). [*] Extended fireboxes to improve efficiency and power led to the 4-wheel trailing truck. Some roads even tried a booster engine on the trailing truck to aid in starting heavy trains. [*]Troughs between the rails holding water were tried so a loco could scoop it up on the move. [*]Automated stokers were developed. [*]Driver sizes varied depending on whether a given loco was designed to haul passengers or freight. [/list] 
I'd be a little skeptical in this case that tradition played a huge role (though it may have played a small one). Instead I'd lay my money on engineering and logistical considerations.


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## lownote (Jan 3, 2008)

I see your point Dwight and you are probably correct. But the other night I was teaching a book on Frederick W. Taylor, the "Father of Scientific Management," who gets his start in the 1880s at Midvale steel in Phila. where they put steel tires on loco wheels. What's striking is that as late as 1910 there are still lots of machine shops where no one has any formal training, and lots of places where "the rule of thumb" is more in place than any objective scientific or engineering standard. Taylor was being hired to look at shops where they have no real standard for, say, lathe speed or the kind of steel or shape of cutting tools, they just do it the way they've always done it. It's interesting how late that stuff develops even in major industries.



I'm not saying the "rule of thumb" isn't effective or maybe even better, I'm just thinking that maybe Americans started one way and then the infrastructure developed to support that way of doing things. It could also be that a saddle tank makes boiler maintenance harder, or maybe it puts too much weight on the drivers.

Not trying to be argumentative, just interested. Live steam got me thinking about it. Watching pressure drop when I pump water through the goodall valve. If the fake saddle tanks on a ruby actually held water, it'd be both an insulator and preheated for the boiler. If an "Ida" had a real saddle tank, it'd be even more so I assume.


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## John Roppel (Jan 2, 2008)

I could be way out on this one. 
does the injector use the temp diffrance between the steam and feedwater to work? just a thought.


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## Dwight Ennis (Jan 2, 2008)

does the injector use the temp diffrance between the steam and feedwater to work?
Not really. It works via the Venturi effect - steam moving at high velocity through a restricted passage imparts some of its velocity to the feedwater which it touches, thereby forcing the water through the injector and into the boiler through a check valve. However, the steam also imparts some of its heat to the water as well, but that's more sauce for the goose.  

Mike - for 20+ years I did mechanical design and drafting at the company I work for. Anytime I did something differently or in a new way, it was more prone to manufacturing error because the guys in the shop are used to "doing it this way because we've always done it this way." Sometimes the way it has always been done isn't the best way in a given application. So that kind of inertia is still present in today's business and manufacturing as well. 

However, despite it, our company still evolves its manufacturing processes and products, and most of this happens in engineering and design, and not by the work force on the floor. I suspect similar processes were at work in steam locomotive manufacturing. 

The fact that tank locomotives were tried at all demonstrates that they were given due consideration, and in certain applications were fuel and water were readily available nearby, and where turning locos was a consideration or in switching applications where visibility were important, tank locos were employed. Where their built-in limitations made them inferior to a normal loco-tender combo, the latter was employed. There were instances where locomotives were originally built as tank locos and later converted, the tanks and fuel bunkers removed, and a tender added. That would cost money and was, I'm sure, done because eliminating the tank locos limitations and the performance gain of the conversion made long-term economic sense to the company who owned the loco.  

BTW, the Ruby was originally designed as a tank loco because it was less expensive to produce and could be sold for less than a Ruby/tender combo (which, btw, sells for considerably more than just a plain Ruby). Economics wins again. hehehe


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## dmk092 (Jan 5, 2008)

I dont know about that. As far as the PRR goes, the E6 atlantic and all subsequent pennsy steam power was engineered from the ground up rather than designed according to rule of thumb. Courtesy of wikipedia, heres a picture of a stock k4: 

http://en.wikipedia.org/wiki/PRR_K4s

Any tank would require downsizing that boiler considerably. Also, remember, North American railroads were always constantly increasing engine and boiler size to handle ever increasing loads, while british and continental engines didn't have quite as demanding work loads.. Consider that by the time WWII started, most major American railroads had shifted to using 4-8-4 northerns for heavy passenger duty, while in the UK 4-6-2 pacifics sufficed until the end of steam. American trains tended to be longer and heavier and covered longer distances. As a result, a decrease in boiler capacity was probably not a design constraint many engineers wanted to make. 

Also, "tradition' would have had a bigger impact on the UK rail system than the American one. In the US, there were three major manufacturers that competed with each other for contracts with all of the class 1 railroads, and few railroads manufactured their own engines (PRR is an exception). As a result, any tradition would have been superceeded by a need to compete, and as a result US manufacturers tended to be fairly practical. In the UK, each of the Big Four manufactured engines for their individual company. As a result, in the UK you do see "tradition" and ego play a large part in engine design. You see things like the LMS small engine policy or axle boxes on the LMS garratts, policies that were carried through because of the individuals involved instead of actual engineering considerations(to be fair, Im sure US steam has its fare share of egotistical engineers pushing through bad designs, but Im not knowledgeable of the subject). Thats my unqualified two cents anyway 



**braces for the fact that Im about to be flamed for criticizing british engine design (for the record, I love the Gresley pacifics even if the third cylinder had a tendency to overheat)****


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## SteveC (Jan 2, 2008)

I don't know that I'm very knowledgeable, but everything that I've ever read in what might be considered as primary evidence (i.e. locomotive manufactures documentation, railroad engineering periodicals, various railroad documentation, car builders cyclopedia, locomotive cyclopedia etc. etc.) and the one single thing you can count on is that the primary thing considered was 'how much did it cost to move freight or passengers per mile', anything that could be proved via practical application to improve that would be implemented.

The only place that I've encountered where the truth was really stretched, was in the attempt at proving the belief that narrow gauge railroads were more cost effective.


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## dmk092 (Jan 5, 2008)

blah. I need to apologize. Reading my last post I realize I was ranting a little and came off a little too strong. In any event, Im far from an expert on the subject, but what I meant to say is that in UK while of course everyone is trying to meet the bottom line, you tend to see design philosophies that are the product of individuals instead of large faceless companies, (Maybe the same thing happened in the US, but the individual engineers never achieved the renown they did in the UK) so perhaps you could make the case that British engine design was influenced by tradition in as much as cheif engineers had their own preferences. 

A possible example of this might be Gresley conjugated valve gear..during WW II it appearantly maintenance was an issue, so Gresleys successor (Thompson, who by some accounts had an ongoing feud with Gresley) decided to remove Gresleys valve gear from some of his pacifics. He was certainly trying to reduce costs, but some might say his decision to remodel Great Northern (the A1 prototype) was colored by his personal feelings about Gresley. 

Information pulled from this website: 

http://www.lner.info/locos/A/a1_1.shtml 

Then again, please correct me if Ive misspoke, Im not knowledgeable of this subject.


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## Dougald (Jan 2, 2008)

In the UK, if the loco assignments are looked at, the longer runs with fast passenger trains were invariably assigned to the larger non tank engines. This was for the same reason that tank engines were not widely used in the USA. The tank engines did not produce enough steam due to limitations in the boiler and firebox design imposed by the water saddles.

In North American practice, only locos which were used to handle small trains short distances were candidates for tank locos. Geography alone meant that most locos on most trains would get far away from fuel and water sources ... hauling a tender behind a tank loco negated its primary benefit while it still had its primary disadvantage of low steam production.

On the subject of preheating water before injection, most modern North American locos (say from USRA designs on and particularly super power steam) had feedwater heaters ahead of the injectors. Elesco was the type universally in use in Canada and, while broadly used in the USA as well, some American roads favoured the Worthington or Coffin heaters. In comparison, warming the water in a tank loco saddle would not get it to the desired temperature while also serving to cool an already too small boiler.

Regards ... Doug


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## Totalwrecker (Feb 26, 2009)

Across the western desert, towns sprang up every 100 miles, mostly to have a water tank for the Railroad. I suspect the bigger the tank the smaller the drivers, so making that 100 mile gap would have been a problem. 
Loggers loved 'em for the high adhesion and braking power hauling huge timbers down from the hills, but they usually made up trains, the bigger boys to hauled to town/lumber yards... 
If you run 20 miles and reach the end where more fuel and water are available, refueling only takes so much time, but if need to stop mid run and find water/fuel it becomes a more time consuming ordeal. But short runs, great pulling power and switching were doable with the water/fuel on board. Longer runs required a larger water/fuel supply and continuing innovations meant boilers were growing in diameter and length and the tender became SOP. Also to consider; Do you want your crew working or refueling? Take off the tanks, slap a tender on the rear and the crew can get a lot more work done.... 
?


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## Mik (Jan 2, 2008)

Couple or three reasons from off the top of my head. 1. side or saddle tanks raise the center of gravity even further than the heavy boiler alone, making a locomotive even less stable at speed. 2 Visibility restriction around that honking huge tank could prove problematic. 3. Your preheating water idea is exactly wrong. Pumps will move warm water, injectors start to get REAL finicky after the feedwater gets above about 80 degrees and most won't work AT ALL if the water is above about 110-120 (you can just trust me on this, ask ANYBODY who's ever had to pour their water jug over a hot injector to get it to pick up, google the physics, or even call Penberthy... they'll all say the same thing).4. The additional weight on the drivers required heavier springing, which means they not only ride rougher, but pound the rail more.


From an engineering (design, not operating, but maybe that too) standpoint tank engines were a necessary evil (because of cramped conditions or sharp curvatures) rather than ANY kind of design ideal.


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## Schlosser (Jan 2, 2008)

Think of how nice a tank engine is in moving a dead locomotive into the round house for repairs. It can ride the turntable with the dead engine because it is SHORT and thus can move it into any stall rather than just the one directly across as it moved onto the table.

This shortness might also be an advantage in switching.

Art


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## George Schreyer (Jan 16, 2009)

A tank engine has only on over-riding characteristic that made it useful in some situations. It was short. In confined yards, industries and on docks, this was important. 

Tenders were not particularly expensive and were easy to manufacture so cost was a lesser concern. Weight on the drivers was good for a tank engine, but the weight would go away with time so it could not be depended on. The smaller boilers were also not a problem as it doesn't take a lot to switch a few cars at a time or pull a couple of commuter coaches. 

Many tank engines had a trailing truck, not because it was needed to support the rear of the engine, but because the engine would track better in reverse, an important characteristic for a push-pull commuter line. Switchers typically did not have leading and trailing trucks, just like their tender equipped bretheren. Visibility to the rear was good in tank engines, handy for switching. Many tender engines were equipped with slope back tenders to improve rearward visibility as the loss of tender volume didn't matter that much in a yard. 

The relatively small water tank and fuel load was not a big deal if you are switching in an area of only a few acres. Fuel and water were always close by and there was lots of down time to refuel/rewater in. 

Commuter services also used tank engines as they also had short runs and time to refuel and rewater often.


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## Torby (Jan 2, 2008)

Always wondered why the sloped back. Thanks!


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