A brake tender is a tender that is heavy and used (primarily) to provide greater braking efficiency.
The largest steam locomotives are semi-permanently coupled by a drawbar to a tender that carries the water and fuel. The fuel source used depends on what is economically available locally. In the UK and parts of Europe, a plentiful supply of coal made this the obvious choice from the earliest days of the steam engine. Until around 1850 in the United States, the vast majority of locomotives burned wood until most of the eastern forests were cleared. Subsequently, coal burning became more widespread, and wood burners were restricted to rural and logging districts.
By the mid-1800s, most steam locomotive tenders consisted of a fuel bunker (that held coal or wood) surrounded by a "U" shaped (when viewed from the top) water jacket. The overall shape of the tender was usually rectangular. The bunker which held the coal was sloped downwards toward the locomotive providing easier access to the coal. The ratio of water to fuel capacities of tenders was normally based on two water-stops to each fuel stop because water was more readily available than fuel. One pound of coal could turn six pounds of water (0.7 gallons) to steam. Therefore, tender capacity ratios were normally close to 14 tons of coal per 10,000 gallons of water.
The water supply in a tender was replenished at water stops and locomotive depots from a dedicated water tower connected to water cranes or gantries. Refilling the tender is the job of the fireman, who is responsible for maintaining the locomotive's fire, steam pressure, and supply of fuel and water.