Two examples of cog rail systems: The top postcard (thanks again Jan and Rick) was postmarked 1912. It pictures the Pike's Peak cog road steam engine with a tilted chassis. The lower postcard, from my grandmother years ago, is more recent (vintage 1940-50's) of the same track with an electric diesel engine similar to the one still used today (click here). In the bygone age of steam-powered locomotives the boilers needed to remain nearly level else the boiler would overheat and explode. This is why the engine is tilted in the top postcard. (See more on cog and rack rail systems and tilted boiler steam engines here.)
Cog rail trains are able to ascend and descend grades of 25 and more. This fact makes them especially suited for mountain terrains. Conventional rails for trains are commonly moved along grades of less than 1%. Though higher gradients exist on some rail lines, the pulling power is greatly reduced between 0.5% and 1%. This is why railroad tracks often make long cuts in hillsides or make long circuitous approaches complete with tunnels through mountain passes. (See my previous post on the Canadian Pacific Railway (here).