hot whistles

Could you please tell me the meaning of "hot" in the following excerpt of the chapter 7 of The Great Gatsby by F. Scott Fitzgerald? Does he employ it in a metaphoric sense?

The next day was broiling, almost the last, certainly the warmest, of the summer. As my train emerged from the tunnel into sunlight, only the hot whistles of the National Biscuit Company broke the simmering hush at noon.

Thank you.