"personification".)
He writes about the steam as though it were being thrown out by someone using a shovel. This gives the impression that the locomotive is alive. When steam engines were in use on railways the drivers and firemen often used to talk about them as though they were alive. Compared to modern diesel locomotives they needed more skill to operate and each engine tended to behave differently - like a human being, or an animal, it seemed to have personality or at least individuality.
The expression "shovelling" also conveys the image of hard work. It makes the reader think about how much power is present in the engine as it pulls a heavy train up hill.
The next line also contains personification.
Snorting noisily as she passes
"Snorting" is a word usually used of animals, such as horses, so that Auden is comparing the locomotive to a hard-working horse. Indeed the steam locomotive in its early days was often called "the iron horse".
A steam locomotive is a machine, made by men to help them. Everything about it can be described scientifically in prose. Nevertheless, when they were first introduced, they caused such changes in people's lives, and were so big and noisy that they were difficult to think about solely in scientific "concrete" terms. Before there were steam engines the largest source of non-human power was the horse. A steam engine has the power of many horses. (Engine power was originally defined in "horsepower".)
All these thoughts are provoked by the word "snorting".
Metaphor in prose
Metaphor can also be found in prose. It may be used in novels, and even in political speeches. Politicians often use military metaphors. They talk about "attacking" the problem of poverty. It is easy to have a picture in the mind of soldiers fighting to surround and enter a town. Is the metaphor appropriate here? Is poverty like a town to be attacked? If poverty is something more difficult to change than it is to drive an enemy out of a town, the use of military metaphors may give people a false impression. Or it may be a sign that the person making the speech has not thought seriously about the meaning of what he is saying. It may even mean that he has no intention of doing anything about the problem but wants to sound energetic. A poet should be more accurate in his speech.
Have you read Animal Farm by George Orwell? If not it should be on your list of books to be read as soon as possible. Animal Farm is a novel which is a kind of metaphor from beginning to end. Orwell himself called it a Fable. It is the story of some animals on a farm who have driven away the human beings and then run the farm for themselves. It's an interesting story even though we know it could not happen. But of course Orwell was not really writing about animals. His purpose was to show how human beings behave. He was really writing about political revolutions and the way that the things people want often don't turn out the way they expect.
African traditional stories about animals are also fables. These animals too stand in for the different kinds of people and the different ways in which people behave and the human mind works. The earliest Fables of this kind in written form are those by Aesop, written in Greek more than 2500 years ago. These animals too show metaphorical language in action.

There is an ancient collection of tales, possibly beginning in India called Kalila and Dimna. These too are about the deeds of animals, but from which we learn about friendship and loyalty - and betrayal.

Simile
Simile is a special form of metaphor. Simile compares one thing with another. The word comes from the Latin for "like". The English word "similar" is related.
Example:
Her face was like the moon.
Her lips were like cherries

Here is a prose passage, packed with similes and metaphors. It is from an English translation of a Persian Classic, Yusuf and Zuleikha by the poet Jami.
The bow of love fires its arrows everywhere, and they cannot be warded off with the shield of rationality. Once an arrow has hit home to the heart, it gives its presence away by a host of signs. How true the saying is: There are two things which cannot be hidden, love and musk.
By concealing her love, Zuleikha had planted the seed of sorrow in her breast: and this was germinating and sprouting upwards into view in spite of her.
Sometimes she would weep; - and each teardrop falling from her lashes revealed her secret. Sometimes it was her sighs rising from her burning heart like smoke into the sky, which gave her away. Her cheeks, once so rosy, were now like yellow tulips, since she went without food and sleep.

Metaphors:
the bow of love
the shield of rationality
the arrow has hit home
the seed of sorrow germinating and sprouting
each teardrop revealed her sorrow (that is, the onlooker noticed her tears and inferred that she was in love)

Similes:
Her cheeks were like yellow tulips
Love and musk (musk - a perfume which is easy to detect because of its strong smell)
These metaphors and similes are commonly used in Europe and Asia when talking about love. Falling in love is so mysterious that the ancient Greeks thought it was caused by a god - they called him Cupid - who fired an invisible arrow at the person. Falling in love is rather like the inspiration which causes a poet to compose a poem.
The bow of love
You may find in other poems the expressions Cupid's bow, Cupid's dart, Cupid's arrows.
In traditional philosophy the heart is said to be the place of emotions as the head is of thoughts. Therefore Cupid's arrows must hit the heart. (This heart is not necessarily the same as the blood pump.)
The shield of rationality
This is the metaphor which says something like: Our reason may tell us that it is painful, or foolish, to fall in love, but although thinking can protect us from some kinds of danger, love is so strong that thinking about it will not prevent it. The metaphorical method of saying it is shorter and better.
Here is another passage from the same book. It describes how Zuleikha falls in love with Yusuf, even though she only sees him in a dream.

It happened one sweet night, sweet as the dawn of life, full of the exhilaration of youth. In the palace bustling life had drawn in its feet beneath the hem of its robe; and nothing stirred: only the stars had their eyes open. Night, like a thief, had robbed the guards of all sensation. The dogs had their tails wound about their throats, as if to stifle any bark.
Sweet sleep weighed down Zuleikha's eyelids. The silky threads of her tousled locks traced pictures on her rosy cheeks. The eyes that see the form of things were closed in sleep; but those other eyes, the eyes of her heart, were wide open: and with them she saw a youth; or rather a pure spirit, a radiant apparition from the realm of pure light, eclipsing the houris in the garden of eternity.
His form was like a slender tree; his nobility of hearing put even the proud cypress to shame. His hair, hanging in chainlike ringlets, was enough to fetter the reason of the wisest mortal. Sun and moon bowed down before the radiance of his brow. His eyebrows were like bows, shooting the arrows of his lashes into every heart. When he smiled, his pearly teeth flashed between his ruby lips like lightning at sunset. The strength of his arms contrasted with the slenderness of his waist.
p14 Yusuf and Zuleikha by Jami translated by David Pendlebury (Octagon 1980)