to a significant degree - according to the dictionary.

It is often another way of saying way of saying 'very' - or simply a synonym for the sake of variation in many cases.

you might also say 'really febrile metaphors'
-- in any case, it emphasises whatever follows when used in this way, without going to absolutes.

(not 'extremely febrile metphor' for instance.)

I understand it as meaning that he consoled himself by taking flights of fancy. He was resigned to his fate, but had to fill in the time somehow. Instead of screaming and panicking, as some people would, he used the 'metaphors' to occupy his mind.

I think most people would be *rather* febrile, if not in a state of panic, if they were really in a train rushing to inevitable death.

The slightly ironic or unsympathetic tone of the sentence suggests that maybe it was not really as bad as all that, or the author does not sympathise with this character. Perhaps it is preparing the reader for some miracle that saves the situation.