Colombia is supposed to be quite nice. It appears to have an extensive rail infrastructure. Unless you're particularly wanting to terminate yourself or others, avoid the Darién Gap border.
Problem with the South American area is crime and language. Not much info at all on their railways, mostly because they are a dying passenger transport.
Saying that, I think governments try and scare us to stay away from these countries for other motives.
I think you're paying £££ for your safety. Solo travel isn't a great idea.
http://www.turistren.co/ - 3 hour trip in each direction, using steam.
