musskopf:
Just break the problem into small pieces... Make the data be transmitted to a node not very far away, once this node receives the data, it retransmit it forward and so on.
While I am a great believer in breaking problems into manageable chunks I doubt very much that this proposal is a practical option for this application.
Apart from any other considerations I suspect the OP needs a solution that works reliably out of the box without the need for experimentation. There are so many potential variables and obstacles that it would take a very long time, and cost a lot of money, to prove that a solution works with sufficent reliability. For example, how many train journeys over how many different routes would be needed before you could have confidence that the system works?
IMHO piggy-backing on an already proven system is the only practical option.
...R