The opening view of the firebox, as has been noted by others, is heavy with possible meaning. The contrasts between shadow and light, the black train and the white steam, are striking. I also noticed the whiteness of the cigarettes -- the only things in the cab that weren't covered in grime. They really stood out. I've recently watched movies in which airplanes (Dr. Strangelove) and submarines (Ice Station Zebra) served as co-stars -- "exotic" technology, loaded with power and potential danger. Those were military machines, but I get a similar sense from the train here.
I think what's even more ominous than the line itself is what follows: As the woman carried the laundry up the steps, we heard the children resume their song -- they ignored her plea to stop as, we can assume from her chat with Elsie's mother, they have done many times before. Elsie's mother's remark makes us in the audience listen hard to hear the song again -- and to see Mrs. Beckmann's (presumably relieved) response to it -- but all we get is an interval of silence. With our alertness heightened this way, the chime of the cuckoo clock is startling and disturbing to us, even though Mrs. Beck