Selecting Motive Power for the Model Railroad

The one thing that likely draws most people to trains in general, and model railroading specifically, are the locomotives.  The engines are the heart and soul of the railroading.  When you first get into the hobby it is the locomotives that will get you hooked but it might also me an area of anxiety.  Here are a few things to consider as it relates to locomotives for your model railroad.

1. Era You are Modeling - Not everyone is concerned about the era their model railroad is set in but if you are going for 'realism' with your railroad it is important.  The earliest period of railroading is all steam power.  If you are modeling the 'modern' era (about 1970 to current) you will be doing diesel power.  The period from the late 40s to the late 60s is a transition period where both steam and diesel power might be running side by side.
2. Scale or Gage - Do you want to model in N or HO or some other size?  Now is the time to figure this out as once you start down one scale it is very hard to change your mind.  
3. Type of Operation - The type of railroad operation you plan to model will also influence your selection of motive power.  Long haul operations involve very big trains with very big locomotive and frequently more than one on the train.  If you are doing local industrial switching then you will need smaller and few locomotives.  These principles are true for both the steam and diesel eras.
4. What Railroad? - The actual railroad you are interested in modeling will influence the specific types or models of locomotives.  Many railroads, especially in the the steam era, had special order locomotives and no other railroad used those particular models.
5. Power and Control System - The power and control system you plan to use - DC or DCC - will also impact your selection of models.  Not all locomotive types are available in both types of control out of the box.  Many more will be available if you are comfortable making minor modifications to the locomotive to adapt them to your control system.

I know what you are thinking - why go through all this when there are so many cool models out there?  You certainly don't have to do any of this.  You could just buy whatever catches your eye and that is a valid way to pursue the hobby.  I, personally, need more structure to by purchasing plan specifically because there are so many cool models available.  Modern locomotive models start at about $100 and go up to $300 or more.  Unrestrained purchases of locomotives would destroy my ability to do anything else.  It really is best to have a plan and try and stick with it.  There is a lot of other things you need in order to build your model railroad.

I am fortunate in that I know the era and the railroad I want to model.  There are lots of models for the Southern Pacific during the late eighties and early nineties.  It also helps that I have a good assortment of DC motive power and a few that are ready for the DCC upgrade.