Alan, what happens when you get to the end of the electrical rail line?  is the rolling stock compatible with conventional tracks and locomotives?  I live in a relatively isolated area that is less likely to be electrified, but does have a small Amtrak station.  As long as the train could switch engines, this would be a manageable problem.  
The only difference is the locomotives.  Either change them or have diesel-electrics (today 99+% are diesel electrics) that can switch fromm on-board diesel generation to grid by raising a pantograph.

In other nations with mixed electric & FF rail lines (UK, Russia, India) changing locos is the normal practive.

Long distance trains through rural areas are good candidates for electrification.

Best Hopes,

Alan