谁能帮我分析下这个句子是什么从句?The primary reason was skepticism

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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.
======以下答案可供参考======
供参考答案1:
that后引导的从句是表语从句，修饰skepticism;
so adj. a sth.(or sth.)是固定结构；
整句翻译为：
根本原因在于对这点持怀疑态度，那就是：已建成的穿越如此具有挑战性的、人烟稀少的、广泛延伸的荒原（或沙漠）、山岭、半干旱平原地带能否支付建设期利润。