In the sentence "Historically, energy sources for train locomotives have been horses, steam, gravity, electricity, and gas," "horses, steam, gravity, electricity, and gas" is a compound what

The words are not compound in nature. They are simply a collection of nouns all of which historically were a source of energy for train locomotives.

Explanation:

In order to qualify for a compound noun, a word must be made up of two or more nouns. For example, "tooth"  and "Paste" are nouns all by themselves.

They can be put together to form toothpaste. Another example would be "web" and "page". Together, we have "webpage".

The words indicated above don't pass for adjectives, verbs and or adverbs in the context in which they have been used.

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