I've found that empires with heavily railroaded infrastructures - particularly on foreign continents - are virtually impenetrable due to their ability to have their entire army on top of your invasion force within 2 turns. You can't transport enough soldiers to survive any counter-attack, even if you do manage to take a city on your offensive. I know you can bombard with ships - which I believe can destroy rails - but somebody told me that air units can't destroy rails? It seems like that would be the best use for them. How do people deal with this, particularly in eras before flight (if air units can destroy rails)?