Scrapyard Diesels

I am planning to build a small diorama featuring a Class 35 Hymek languishing in a locomotive scrapyard. As part of the heavy weathering process, I intend to include large areas of rust.
Can anyone confirm whether the entire body of the locomotive would have been made of steel,
or just certain parts? Clearly, this will have a bearing on my planned ‘rust’ treatment.

Thanks,

Paul