Because while the railroads hadnt connect the heartland from east to west it was still conneted to the rest of the world by it's massive rivers which allowed trade from Southern ports over the sea. At first trade was mostly transatlantic in America but it's transpacific now and happily most the goods on the pacific coast are new economy ones too. Usually when global trade patterns change like that this it would cripple a nation that has bet it's long term future on one set of particular routes but this did not happen here.
Indeed Britain was in the more advantageous position initially but then Americas advantages were more longterm.
You had lots of resources and plenty with room of to expand with a connected heartland and a Pacific and Atlantic coastline to allow room for economic flexibility.
Allied with Britains need to defend disparate colonies for resources and the later crippling of Britain in war it is easy see why America is now the boss of Britain.
They say one of the reasons they unleashed the Tans in Ireland was because large numbers of British troops were tied down in the new colonies it had seized from the Turks after WW1.
This diastrous policy in essence shows how weak Britain really was even at the imperial core, they lost a part of there core terroritry because they tried to hold some desert abroad.
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