Large fuel tanks, such as those on locomotives, or storage tanks have a significant amount of freeboard, (air space above the liquid to the top of the tank) when going through operating cycles. Most operators with fuel tanks will not take daily deliveries to keep tanks near full (sometimes they will depending on what fuel prices are doing) so the freeboard will introduce humidity into the tank from the outside air. Since oil is lighter than water, if condensation does form within the tank, the water will settle to the bottom. This is particularly common with stationary tanks, although I imagine locomotive tanks do not undergo the sloshing a truck or car tank does. Anyhow, over time a layer of water and sediment, known sometimes as "slosh" forms in the tank. So long as it is not excessive, its easier to just let it be, rather than dispose of large amounts of contaminated fuel, and then deal with the inner surface of the tank. In stationary applications, it is common to have the fuel pickup 1-2" off the bottom of the tank to permit slosh and not suck it into the outflow lines. This is even more the case with waste oil furnaces and boilers, sometimes there will be up to 5% slosh capacity in a tank to allow antifreeze, coolant, condensatio and water to settle out and not get introduced into the burner.

Also related, this is one of the nasty things that Ethanol fuels do in the metal gas tanks of older vehicles. Ethanol is hydrophillic and absorbs water. It will then settle to the bottom of the fuel tank, being heavier that the gasoline and it will corrode the inside of the tank. This is in addition to all the other terrible things ethanol does to older fuel systems like corrode aluminum carburators and fuel lines, eat away at rubber hoses and seals etc. Buy E0 gas if you have a pre-1995 car.