Good points that you brought up. Always interesting for me to hear how things are done in the West and mid-West where lots of thing are spaced out very far - my experience is mainly with East Coast style design, dealing with very large concentrations of load and relatively short distances (compared to the West at least, as far as I know).
I'm sure being 'green' for the sake of being 'green' plays a large role. I would also say that existing infrastructure played a large role. Coal has been used in generation forever, and many railroads have been in service for a very long time as well (definitely when coal came about). I personally cannot name any new railroads constructed in the last 15 years, not where I live. If the railroads are already there and they are cheap to use, why not use them?
Cheaper does not mean more efficient. If you've ever had to deal with long range planning or budgeting or anything similar for a company, you know that board members, VPs, and C-suite staff only care about the bottom line at the end of the day - it outweighs anything else. Whether or not more usable 'power' could be moved by transmission lines vs rail, I guarantee the cheaper option at the time was the one that was chosen.
I'm actually not sure what conductors were used in older transmission lines. I'm sure much if it is ACSR, which is relatively cheap and strong. Any newer lines are likely AAC, as aluminum is an excellent conductor of electricity. I'm sure some transmission lines use a composite conductor or some form of copper-core conductor as well. I am not familiar with any superconductor transmission lines so I can't speak about that.
One other concept is HVDC transmission, which uses DC as opposed to AC. The main benefit I know is reduced cost after a certain break-even distance with AC transmission - this makes them great for super long distances. Because the current is non-oscillating, it is typically a little more efficient to transmit at long ranges. The only reason AC has been used all through history is because it can easily be stepped up or down to different voltage levels. With the development of the semi-conductor and as technology progresses, it is getting easier to 'transform' DC power to different usable voltages with low losses.
One thing you're seemingly assuming is that the classical power supply model will remain the same forever - it has already changed. This is an issue I deal with on a day-to-day basis coordinating existing protective equipment and load centers. DERs have exploded in recent times - think of it as smaller-scale generation that is dispersed throughout existing load centers. This includes rooftop solar, wind, municipal water reservoirs, municipal flywheels, on-site geothermal storage, etc.
Instead of power being generated "off-site" and moved "on-site", "on-site" generation is rapidly growing. The system was not designed for this, and we are actively trying to make major improvements to accommodate.
Hope this clears a lot of things up. Happy to discuss further
