Real railways are not built to look nice, but to serve a purpose - move goods and carry passengers around. So, the first thing you should find out is what is the purpose of the railway you are going to built. What commodities are to be produced and consumed. In what quantities and over what distances goods are to be transported. By how long trains. Answers to such questions should determine the distribution of the industries, as well as general layout of your route.
If you like your route to be interactive and playable, limit available choices and stick to achievable processes. Sometimes less is more. Too many industries will require a complex rail road system and they will clutter your map. AI drivers are not very good in servicing elaborated networks. Know their limitations and plan accordingly.
An example - currently I build an imaginary route, looking like set somewhere in Pennsylvania and servicing a small car industry. I have two lines, one double-track 14 miles long, another single-track 12 miles long, intersecting in the middle. Along these lines I have five distinctive industries - a coal tipple, a power station, a steel mill (consisting of three industry tracks: coal/iron ore bunker, strip mill and plate mill), a car factory and an inland, river harbor. So, in this case it comes to about 5 miles per one industry. Two passenger trains and two goods trains in minimal configuration.
Try to experiment a bit and this way you will find out what works for you. Good luck!