Transferring production between cities

[Corbeau]

In Civ, city production depends solely on the neighbouring tiles. However, in RL things work very differently: the location of industrial centres depends on many other things. If you have a coal or iron rich area, it benefits the whole country, not only neighbouring city. Industrial products are transported to other cities and the nation's industry depends on organisation of routes, not which city is near which coal or iron mine. So I would like Freeciv to reflect this and allow cities in unfavourable locations to have good production if it is organised well (and sufficient tech level is reached).

I can think of three ways of doing this.

1. This method is already available, but there is a problem. First, the method: cities fulfilling conditions (export-import buildings present) and connected with trade routes "exchange" shields simply by one city having +X shields and the other -X shields. The problem: trade routes can't be cut which removes flexibility crucial for this to work. So, the change that would need to be done is simply making it possible to cut a trade route through user interface.

2. Interface item that simply says "transfer X production from A to B". No idea how that would be implemented, any idea is good, I guess, but as long as it works, nobody has the right to complain.

3. A Caravan-like unit that transports X amount of shields, "helps build" whatever is being built and works for everything, not only for wonders.

So, which of these methods would be most neat and/or which would be the easiest to implement?

EDIT: Turns out, #3 is already available.