The measured motion of their shadows on the walls, was the substitute (关系代词that省略）Coketown had to show for the shadows of rustling woods.

参考译文：Coketown能展示给人们的不是沙沙作响的树林映在墙上的影子，而是机头映在墙上缓慢而有节奏的动作。

以上句子语法结构分析如下

主语：The measured motion of their shadows on the walls

系动词：was

表语：the substitute

修饰表语后置定语：Coketown had to show for the shadows of rustling woods.

定语从句语法结构分析如下

名词：the substitute

修饰名词定语从句：that Coketown had to show for the shadows of rustling woods.

定语从句主语：Coketown

定语从句谓语：had

定语从句宾语：that(在定语从句作宾语可以省略）

定语从句目的状语：to show for the shadows of rustling woods.