the on rushing train gives carrie what impression and why?

"They were nearing Chicago. Signs were everywhere numerous. Trains flashed by them. Across wide stretches of flat, open prairie they could see lines of telegraph poles stalking across the fields toward the great city. Far away were indications of suburban towns, some big smokestacks towering high in the air." (Sister Carrie/ Chapter One)

I'm not sure what your teacher was asking here either; the flash of the train is a combined visual with the other signs that signal the "big city," and the arrival at her final destination. It's a new world and one she's unaccustomed to, but she focuses on the "lights" and the "flashes" as if they belong to her.

"To the child, the genius with imagination, or the wholly untravelled, the approach to a great city for the first time is a wonderful thing. Particularly if it be evening—that mystic period between the glare and gloom of the world when life is changing from one sphere or condition to another. Ah, the promise of the night. What does it not hold for the weary! What old illusion of hope is not here forever repeated! Says the soul of the toiler to itself, "I shall soon be free. I shall be in the ways and the hosts of the merry. The streets, the lamps, the lighted chamber set for dining, are for me. The theatre, the halls, the parties, the ways of rest and the paths of song—these are mine in the night." Though all humanity be still enclosed in the shops, the thrill runs abroad. It is in the air. The dullest feel something which they may not always express or describe. It is the lifting of the burden of toil." (Sister Carrie/ Chapter One)