The Wikipedia plot summary doesn't do justice to the train car's unbelievable content  -- it was fake ingots made of compressed coal dust, covered with gold-coloured paint -- the credulous protagonists eventually learn this amazing fact after a lot of violence and running around, by smashing them open. They later realize the whole car is 'made of solid gold' after someone throws one of the fake ingots at it, scratching through the car's paint. But the real valuable cargo is a deed to the Brooklyn Bridge, disguised as a gum wrapper!