About a train and a lock

In a world where there are only two towns (Town A and B), the only way to send stuff between the two towns is in a box by train. The problame is that bandaits raid every train and steal everything that is in a box thats not locked (the box is they leave on the train).

How does a person from town A sends an object to a person from town B?

1) everything that goes by train has to be in a box
2)The box cannot be opened by anyway beside the right key (No breaking, cutting, mealting the box. Or any other way of oppening the box beside the key)
3) there is no telephone so the only way to communicate is by letters....that are transfored by train inside a locked box (otherwise they will be stolen)
4)There is no masterkey, every lock has only one key.
5)person A and person B can't leave their towns.
6) the towns are far apart.

*I just put no.6 in there so that people won't replay with answers like "the towns are really close together and person A just goes and throws the package to person B" (and i have heard answers like that)

Good luck