look up the railroad hijackings back in the 20s important documents and money from the mail kept getting stolen the us govt put a few marines to guard the trains and not one piece of mail was stolen after that ..
"Toward the end of 1926, the men of the 4th Regiment had an opportunity for something more exciting than garrison routine. A recrudescence of robberies of the United States mails, featured by a particularly brazen and bloody attack on a mailtruck at Elizabeth, New Jersey, on 14 October 1926, led to arequest by the Post Office Department for the services of theMarine Corps to bring the situation under control. The Marines
had been called upon once before to guard the mails, when a
similar situation had developed in the fall of 1921, and they
had quickly put a stop to the robberies. There had been
virtually no incidents after the Marines had entered the picture
on that occasion, and after they had been withdrawn in the
spring of 1922, the Post Office Department, having provided
itself with civilian armed guards, had been able to carry on
satisfactorily for some four years''
cited from Leatherneck.com