同位语从句
主句:The primary reason was skepticism.
that从句做skepticism的同位语,具体说明skepticism.
that从句主干a railroad could pay a profit.其他的都是用来修饰railroad...过去分词短语做后置定语修饰railroad.
常见的同位语从句先行词是news,idea,fact,promise,question,doubt,thought,hope,message,suggestion,words（消息）,possibility等.
这些都是抽象名词,你的这个例子中skepticism,怀疑态度也是抽象名词.
同位语从句的特点是,你可以去掉先行词,句子仍然成立,没有语法错误.
这里The primary reason was that a railroad built through so challenging and thinly settled a stretch of desert,mountain,and semiarid plain could pay a profit.无错误.所以原句是同位语从句.