Sat May 18, 2013 12:37 pm
Once built, the rails become part of the target country's rail network, so your advantages of rail buildings are to boost development and production in the region you build (useful when you have economic sites there) and strategic mobility if you use the railway to move units through the territory (if you have passage rights). Commercial Agreements allow building, but the main advantages always to be expected from them are enhanced trade success and improved diplomatic relations from the agreement itself and from the trading activity. In multiplayer, using up economic sites with your buildings may be taken unfavorably unless negotiated in advance (remember that production sites for a resource get a bonus for unused sites in the region). The distribution of resources and the incrementally increasing cost of sites and limited force pools for sites mean that mutual foreign investments can be very mutually beneficial.
Railroads are usually welcomed, but they eat coal, as do fleets, and coal scarcities can be a serious problem in game. So, as Kensai in particular regularly advises in these forums, build what rails you need and don't overextend.