Question

谁能帮我分析下这个句子是什么从句？

The primary reason was skepticism that a railroad built through so challenging and thinly settled a stretch of desert,mountain, and semiarid plain could pay a profit.

希望分析的详细一点，谢谢！

Answer 1

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。

Answer 2

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。