I agree, the sentence in your grammar book, as you have written looks incorrect.

"If we don't hurry up the train will have been leaving before we get there." This sentence is not actually correct. It should be: A) "If we don't hurry up the train will have left before we get there." (using the future conditional) OR B) "If we don't hurry up the train will leave before we get there." (using the future)

I agree. You could use "If we don't hurry up, the train will leave before we get there", or, less elegantly, "If we don't hurry up, the train will have left before we get there". But the choice in your example is horrible. I wonder if you need a different grammar book? This tense is little used. And the example the book gives is just silly. Oh, yes: you might well expect a comma after the "if" clause - "If we don't hurry up, the train ..." https://esllibrary.com/blog/future-perfect-vs-future-perfect-progressive https://ginsengenglish.com/blog/english-verb-tense-frequency