If you have ever watched a freight train you have probably noticed at one time or another tank cars in the mix, usually carrying some type of chemical commodity. This type of railroad car has also become more prolific and diversified in recent years hauling increasing types of materials such as food-based products, petrochemicals, and ethanol. However, at one time this was not the case and in many ways the tank car is "specialty" freight car, meaning that its creation came about to haul a specific commodity or commodity type. While the perception has started to change in recent times for many years, even through the latter half of the 20th century, railroads have been quite stubborn about accepting new technologies or any change at all for that matter. So, when new oil companies began to spring up after oil reserves were discovered in the 1860s railroads were, not surprisingly, hostile to developing a new type of car to haul the commodity.
However, on the continued insistence of these companies change did begin to take place. Before this, and with almost every type of commodity (liquid or solid), railroads used a combination of boxcars, flatcars, and gondolas to haul everything from lumber and coal to crude oil and water (by use of barrels). The inefficiencies of hauling liquids this way (barrels meant much less of the commodity could be transported and barrel construction was never seal-tight so they often leaked or were easily damaged in transit) quickly became obvious not only by the oil companies but also other businesses which had their liquid goods shipped via railroads.
The earliest version of tank cars, or oil car as it was known at the time, began to appear in the early 1860s still during the Civil War. Essentially, a standard flatcar was employed with two or three vertical vats placed on top. Unfortunately, the design had several flaws; the most obvious was that it could not hold very much product due to height and width issues. Additionally, however, the wooden vats leaked. By the late 1860s following the war the first true tank car began to take shape, transitioning from an early vertical vat-on-flat-car design to a horizontal iron tank which was mounted to a traditional flat car, roughly similar to what is commonly used today featuring a top dome to load the product as well as a discharge valve at the bottom. This new design allowed for much more of a commodity type to be shipped via one car and quickly became widely used, especially to haul crude oil.
By the 1870s and 1880s tank cars were in widespread use across the industry not the least of which was due to the fact that the oil/petroleum business was growing into a very profitable venture for railroads. Despite the better design the car was still traditionally built with wood at this time where the horizontal tank was fastened to a flatcar. However, as the 20th century progressed the car was constructed of better materials. For instance, with the development of steel and welded seams the car was much stronger and thus could be built larger to haul much heavier loads. Also during the 20th century many more commodity types became available from eatable liquids to various oil products, which required differing types of tank cars to be built (such as those with interior glass-linings to accommodate edible commodities and chemicals).
Despite the widespread use of the tanker by the late 19th century railroads still found the car somewhat of annoyance since it typically could only be used to haul one product. For instance, a car carrying oil could not haul any other type of liquid, such as a food-based product. This led to the rise of privately-owned rail car fleets, especially within the petroleum and chemical industry. Throughout the first half of the 20th century the car's demand continued to climb, first with the introduction of the automobile and then later during both World Wars with a brief decline during the great depression. Today, most tankers are privately owned and as mentioned above carry a wide range of products from industrial to food-based.
Over the last half-century or so the tank car's overall design has not changed significantly, with any alterations within the general design itself to accommodate various commodities (such as differing positions of the dome, walkways, and pressurized/non-pressurized railroad cars). If you are curious as to what a car may be carrying their product is usually labeled on the lower-right hand corner of the car along with other reporting marks (and will say something such as "ammonia," "chlorine," or some type of acid). Given the fact that today there are so many differing types of bulk liquid commodities to carried on the railroads the tanks car will certainly always be one type of freight cars used by the railroad industry for years to come.
For more reading about tank cars consider the book, Freight Train Cars, from Mike Schafer, one of the leading authors covering all corners of the railroad industry (from its history to present day operations). The book gives an excellent general history and overview on all of the common freight car types and if you are interested in learning more about them, or you are simply looking to better understanding their history and development you should very much enjoy Mr. Schafer’s book. If you're interested in perhaps purchasing this book please visit the link below which will take you to ordering information through Amazon.com, the trusted online shopping network.