They built awfully bad ones, by today's standards. Rails weren't prestretched and welded together. Instead, they left gaps between rails to cater for temperature differences (http://en.wikipedia.org/wiki/Track_(rail_transport)#Joining_...)

Modern trains also are heavier and accelerate and decelerate faster than the trains you see in westerns. Both mean that they exert greater forces on the rails.

Finally, safety standards are way higher. That means that material must be of higher quality and tested better, both at the factory and after installation.