Mountains are the most inaccessible terrain in Imperialism but can be the most lucrative.
Long before you can put a railroad or a depot on a mountain (which need dynamite to be discovered), you will have prospected it for possible minerals. As with hills, mountains can yield coal or iron, but they can also hold (sometimes in conjunction with coal or iron) gold or the very lucrative gems.
If your prospector finds minerals, your miner can do a little work to develop a mine that produces (as do forests and fields etc) resources with no further human input except freight cars.