It would be awesome if the map system include a network of rails, kinda like the roads and bus stop that connect everything. (sadly they often not connect all together)
I think it should be an event, like the cargo ship but maybe not all around the map, could be a one way convoy garded by heavy scientist.
As all areas of the map will potentially have a monument connected by a road/rail. The convoy could spawn somewhere and reach one or a few monuments.