Spain has the second biggest high speed train network of the world, with over 3.000 km (many more kilometers being built), just after China.
This 2014, 28M people will have traveled by train, compared to 31.2M in USA in 2012. The goal is to reach 56M in a few years. After many years where the prices were expensive, we have started to liberalize it and prices have go down. Now it's reasonable cheap. You can buy one-way tickets for 30€ for a 1h30 route (like 4h by car). Madrid-Barcelona is one of the most demanding routes and flights can't compete with the high speed train. In just a few years, people have moved from using flights to most of them using the train. It's way way more comfortable.
Of course, the key question is geography. Trains works best for a distance less than 1.000km. In Spain that's the distance from east to west or south to north. In USA there are some areas that make a lot of sense like Boston-NY-Philly-Washington; and California.