Answer:
That the human race is of one distinct species. It exhibits slight variations (in color, stature, mental caliber, and so on) due to climate, environment, and so forth; but it is a species by itself, and not to be confounded with any other.
That the quadrupeds are a distinct family, also. This family exhibits variations – in color, size, food preferences, and so on; but it is a family by itself.
That the other families – the birds, the fishes, the insects, the reptiles, etc. – are more or less distinct, also. They are in the procession. They are links in the chain which stretches down from the higher animals to man at the bottom.
The trains in the stories symbolized the journey the characters underwent to get what they want.
The trains are also the witnesses to the death of the characters. In the story "Paul's case" Paul committed suicide by jumping in front of a train. In the story "A Journey" the husband died in his sleep inside the train while the wife accidentally died when the train reached its destination.
It also sends a message of whatever journey one enjoys in life, he/she will always end up dead or death is an inevitable destination.
I'm assuming you mean tropes, and it's because the original story tellers used three, leading to story tellers after that to copy them and use the rule of three, and so it just became a normal thing.
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