Some simple math:
1) Take an average GP40-2, weighing 260,000 pounds. Decided by 4, the number of drive axles, equals 65,000 pounds per axle.
2) Now, the above example thus makes 32,500 pounds PER WHEEL!
3) That 32,500 pounds per wheel is applying a force on the rail head, about the size of a half dollar!
Think about that.