Those magnets look like the disc and thumbtack type to me.
My son has one of those Brio train sets (not cheap!), and those too have the same type of set up (magnet & pushpin).
I did find some earth magnets. And you are right, they are pretty expensive. They are also flamable, and I wouldn't reccomend placing them inside a childs toy.
Here's the link if you want to compare prices http://www.rockler.com/findit.cfm?page=10713&sid=AF136