The main error of IA is to build massive railroad areas around each cities and connect them (they are generaly too close ...) without protecting the access...

You can easly exploit this error.

Generaly enemy city in front of yours are seriously defended. But far enemy cities are not.

So take some big military units and one diplomate, or a caravan, or a spy, what you want that ignore enemy zone.

Now enter in its railroad network. always move your diplomate first and your armies after. So your armie will not be blocked by enemy zone. and you can merely attack every enemy cities in one turn (blood and guts) ... espacially far and low defended cities.