Quoth the article:

A large exception to this rule are railroads in the United States and Canada, who employ their own police forces to enforce state or provincial trespassing laws. Railroad police have the ability to independently arrest and prosecute trespassers without the approval or assistance of local law enforcement. Further, in many jurisdictions, trespassing on railroad tracks is considered a very severe offense comparable to drunk driving and marijuana possession with fines that may even surpass the latter two crimes.

http://en.wikipedia.org/wiki/Trespass