Try not to paint all Republicans with such a broad brush. Trent Lott, Ray LaHood, and others come to mind, as do governors of VA, NC and other states. We need to be fair and recognize allies on both sides of the divide.
Also Republicans seem to have a particular issue with Amtrak. They have been more supportive of passenger trains in their local domains which are not Amtrak operated. OTOH there have been significant powerful Democrats in a very blue state like NJ who have vehemently opposed addition of new train service, one of them allegedly because it would compete with a bus company that he had interest in. Fortunately at least he has departed this world in peace.
Just like with any other issues, painting with a broad brush will cause one to miss the details and nuances and sometimes lead to hopelessly wrong conclusions about how to advance the cause of passenger trains.
For example the Southern Rail Group which is pushing for restoration of passenger service along the Gulf Coast counts among its supporters several Republicans including a US Senator (according to one of our friends at Amtrak), who played a significant role in pushing Amtrak to complete the study.