I am not sure what is behind the obsession with blaming Republicans.
It was politicians across the political spectrum that became enamored with the automobile and the job producing roads and highway construction they created. From mayors to presidents the decisions of those democrats and republicans changed the USA from a rail using society to a car based one. Many large labor unions also supported highway construction for the jobs it gave dues paying members.
To single out one party and dump all of the blame for the decline of railroads on them is petty election year fodder for the mindless drones that will swallow any garbage put out there.
With that being said, you are lacking one key bit of information on this topic when you blame republicans. Amtrak, the governments hail mary pass to save passenger rail service, was created by law in 1971 under a law signed in 1970 by Nixon. A republican.
http://en.wikipedia.org/wiki/AmtrakThe lack of dedicated rails for passenger service can largely be blamed on the US government closing down rail lines after forming Amtrak. They did it as a budget cutting measure. Democrat and republican bureaucrats decided to run Amtrak on private freight lines to save maintenace costs while paying minimal fees for the track use. They dumped the maintenance costs on private businesses.
In the late 1970s I watched government contractors rip up miles of the express line that ran through my city and surrounding towns that used to carry the Portland Maine or Concord NH to Boston to NYC and beyond trains. The railbed running through my home town used to hold 4 sets of tracks. Today it has only 2 which must now carry over 45 passenger trains and several freight trains per day.
You can take a skimobile from the Massachusetts border all the way to Canada on one converted rail trail in NH, but trains have to switch tracks a dozen times to make the same trip. A recent attempt to reopen a coastal line from Boston to Portland Maine ran into a problem in Newburyport MA where the town had illegally ripped up a mile of track for a new trail system. The problem was found when a train was sent north as part of a track test. They nearly derailed the train when the track just ended.
That is the story across the US.
And the Post Office is buried by an unsustainable retirement plan. Thank god the railroad retirement plan does not suffer the same poor planning.
