Pros and Cons
The benefits of tank locomotives include:
- Bi-directionality: most tank locomotives are capable of running at full speed in either direction. Most tender locomotives are unable to do this, because the heavy tender is not designed to be pushed and may become unstable at higher speeds. Tender locomotives generally require turning facilities, such as a turntable or wye, at each end of the run. A tank locomotive, on the other hand, can simply run around the train and pull it back in the other direction. The crew of a tank engine generally have a better view in the reverse direction than for a tender engine and are protected from the weather.
- Fuel and water add to adhesive weight: the usable tractive weight of a locomotive is the product of the weight on its drivers multiplied by the factor of adhesion. Therefore, up to the limits of the maximum permissible axle loading, and other loading limits, the more weight on the driving wheels the better. In a tank locomotive the weight of its own fuel and water increase the available tractive weight.
- Compactness: A tank locomotive is shorter than the equivalent tender locomotive. This is important in environments limited space for locomotives, for example the headshunt of a run-round loop.
- Efficiency: Many train tanks are designed to be in contact with and be heated by, the boiler. Pre-heated water will reach boiling point faster than the colder water available from a tender. However, excessively hot water can interfere with steam injector operation and is to be avoided.
There are corresponding disadvantages:
- Limited fuel and water capacity: a tender can typically contain far more of both than is available on a tank locomotive. This restricts the range of tank locomotives between fueling and watering points.
- Varying adhesive weight: Use of the fuel and water for the purposes of adhesive weight means that the adhesive weight of the locomotive decreases as they are used up.
- Instability: Water surging inside large side tanks can cause the locomotive to become unstable and prone to derailment, as was the case with the LB&SCR L class 4-6-4T before they were modified.
- Axle loading limits a problem: For larger tank locomotives, it is hard to put much fuel and water aboard without requiring more axles than a rigid frame can handle.
- Limit of boiler diameter: The boiler and water tanks must fit within the loading gauge of the railway being run on. Above a certain diameter of boiler there is little or no room for water tanks to be added and still fit within the loading gauge.
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Famous quotes related to pros and cons:
“Quite generally, the familiar, just because it is familiar, is not cognitively understood. The commonest way in which we deceive either ourselves or others about understanding is by assuming something as familiar, and accepting it on that account; with all its pros and cons, such knowing never gets anywhere, and it knows not why.... The analysis of an idea, as it used to be carried out, was, in fact, nothing else than ridding it of the form in which it had become familiar.”
—Georg Wilhelm Friedrich Hegel (1770–1831)