It’ s a theory （ 主句 ,是主系表结构句子） to which many economists subscribe （定语从句,修饰theory） but （ 转折连词,引导并列句） in practice （介词短语作 状语）it （ 主语） often （ 副词作状语） leaves （ 谓语） railroads （ 宾语）in the position of determining which companies will flourish and which will fail.（ 介词短语作宾语补足语 ）
to which many economists subscribe,是定语从句,修饰 theory ,to 是 和subscribe to 搭配的,介词提到了关系代词which 之前,which 指代theory,在句中作 subscribe to 的宾语.
which companies will flourish and which will fail.是两个并列的宾语从句,作 动词determining 的宾语,这两个which 都是 在 宾语从句中作定语,修饰 companies,第二个 which 后 省略了 companies .