Its a theory to which many economists subscribe, but in practice it often leaves railroads in the position of determining which companies will flourish and which will fail.
句子结构要点：
️ 句子主干: Its a theory but it often leaves
railroads in the position
️ 定语从句：to which … subscribe
️ 介词结构作后置定语：
of determining which … which will fail
️ 介词结构作状语：in the position
翻译技巧：
介词+which结构的定语从句
句子翻译：
这是一个许多经济学家支持的理论，但是在实践中，它经常都会使铁路公司处于决定哪些公司能够成功哪些公司将会失败的位置上。