Good layout!

As for the white space, the answer should be easy to come by. Consider what area you intend to have your railway represent. Think about the areas where your stations would reside. Railways were most often created in order to fill needs- the need for some raw material of large bulk to be moved or some finished material to be sent to market. Now that you know where your railway 'operates', it should give you lots of clues about what sort of things could fill the empty places... If there's an area with farms, then the livestock and produce need to be moved to a processing plant and thence to market. If your interest is chemicals, then a processing plant would take top spot in the showcase, along with petroleum as your raw material (from docks? They're another good 'industry').
Well, you get the idea