Tenders, like engines, grew over time. The earliest were of course not much more than a 4-wheel flat with a barrel of water and a pile of wood. Four-wheel tank tenders were first introduced about 1830 and were the norm for the next 15 or so years. They held around 500 gallons of water and about 1/2 cord of wood.
With the growth in railroad mileage and the introduction of larger engines such as the 4-4-0, these tenders simply were too small, requiring too many wood-and-water stops. A larger tender with six wheels was introduced during the 1840s which doubled the capacity. These six-wheel tenders came in various configurations. Many had fixed wheels. However, the tight curves of many American roads caused these to have tracking problems. This gave rise to the idea of mounting four of the wheels in a truck. Six-wheel tenders remained popular into the early 1850s but again increasing need for greater range (and greater capacity) ultimately lead to the eight-wheel tender, which carried 50% more water and fuel than the typical six-wheeler.
Six-wheel tenders remained popular in some locations for many years. In the US, the Boston & Providence, which didn't have many sharp curves, used fixed six-wheel tenders on their engines into the 1860s. In Britain, fixed six-wheel tenders lasted well into the 20th Century (see the Harry Potter loco!).
The Western & Atlantic specified a six-wheel tender when they ordered the YONAH in 1849. The AMENIA, built for the New York & Harlem at nearly the same time, ordered a larger, eight-wheel tender for the same size loco.
The particular configuration of the YONAH tender reflects the basic weight distribution problem experienced with by six-wheel types. The tender itself is unbalanced, with more weight on the rear truck due to the water tank, compared by the comparatively lighter wood fuel load.
Dave
