The train in the poem "Station" represents the inevitable path one takes through life. The story is told from the point of view of a father saying farewell to his son as he boards a train at the station. The father takes note of exactly where along life's journey his son is and where he is himself.

When our cheeks rest glancing against each other, I feel mine scratchy with beard and stubble, his not quite smooth as a girl’s, harder, a faint fuzz.

The poem is full of sentimentalism and nostalgia. The father realizes that his son is entering adolescence, and he never will return from it. Beyond that lies manhood. The train will take the child to his mother's house. But, metaphorically as the father sees it, it will also take him forward through life. Life moves us all forward along a path, just as the train does.

The train seems to symbolize life and moving through the stages of life.  Literally, the boy is going from one parent to the other, but symbolically, it represents moving through life.  He is moving from pre-adolescence into adolescence and beyond, I think, into adulthood at the end, as the father mentions his moving out of sight.  

The train is a symbol for the process of life that carries the son inevitably into adolescence and adulthood, and away from his father.  The train will take him to unexplored areas of his young life, not only geographically, but intellectually and sexually as well.

The whole poem is about this, whether the train is directly mentioned or not, but you can see the symbolism of the train most directly in these lines:

the huge train waits, crowding the station
with aftermath and longing
and all we’ve never said
to one another. He shoulders
his black dufflebag and shifts
from foot to foot, restless to be off,