Wikipedia is a good starting point for Tank locos. click here
A Few Tank locos used on IR in the attached pics:
1. WT 14005 (of Jha Jha shed) at Bandel (By Rob Dickinson - IRFCA)
2. 2-8-4 Tank locomotive used on the Bhor and Thull Ghat inclines, Circa 1900-1920 (Scanned by me)
3....
more... Illustration of the initial set of 0-4-0 Well tank locos used on the GIPR's Bhor and Thull ghat inclines in the 1860's.
4. XT locomotive Circa 1935.
Tank locos did not have much operating range and they weren't capable of high speed runs or heavy haulage as the amount of water and coal they could carry was limited. However, the lack of a tender meant that they were extremely useful in operating suburban services etc.
They were particularly useful on heavy gradients. For example, if a steam locomotive is going downhill in the regular configuration, then the water in the boiler will accumulate at the front of the locomotive. Now, the fire box is located at the rear of the locomotive near the driver. So, in this case, the water will not be heated properly, and it is also possible that the fire box may be damaged as there will be lesser water available to absorb the heat. So, on heavy gradients, it is always advisable to have the tender of the locomotive on the downhill end, so that enough water is accumulated over the firebox. For this, running tender first is necessary when going downhill and as a result tank locos were preferred for routes with heavy gradients.