Which part is the tank of the tank engine?

Question

Is it the big round (main) part of the engine on which the chimney and the whistle are located, in front of the cab?

Answer 1

It's where the water is stored.  A tank engine has a tank of water located on the locomotive itself.  Steam engines require lots and lots of water to operate.

In a "side tank" engine the tanks are on the sides.  In a "saddle tank" engine, the tank is draped over the boiler like a saddle.

Steam locomotives that are NOT tank engines have the tank in the back of the tender, i.e. not on the locomotive itself.

Answer 2

Water tanks on tank engines are usually located on the boiler, as you describe, appearing somewhat like saddle-bags draped over the boiler.

Answer 3

There are three main types of tank engines:-

(1) by far the most usual (at least in the UK) were 'side tanks'  where the tanks sit on top of the frames on each side of the boiler.

(2)  the second most popular form and used extensively on industrial locomotives were 'saddle tanks' where the water tank is slung across the boiler - like a saddle on a horse

(3) the Great Western Railway specialised in 'pannier tanks' where the tanks stick out on each side of the boiler, without going across the top and without going down to the frame.

Photographs of these three types can be seen at:- http://en.wikipedia.org/wiki/Pannier_tan...

which also has descriptions of less common types like well tanks and rear tanks.

Since writing the above, I've re-read your question. The 'big round part' is the firebox and  boiler and appears on all steam locomotives, tank and tender engines alike.