Simple:
* The train, loaded with coal, lumber, and steel, rumbled slowly through the valley, its powerful engine chugging rhythmically as it traversed the tracks, its long line of boxcars stretching out like a ribbon of industry in the fading light.
More complex:
* As the sun dipped below the horizon, casting long shadows across the vast plains, a colossal freight train, its engine a roaring beast of steel and fire, hauling countless tons of raw materials and finished goods across the land, rumbled through a small town, its whistle echoing in the stillness, momentarily disrupting the peaceful tranquility before disappearing into the distance, leaving only a trail of smoke and the lingering echo of its journey.
Humorous:
* The freight train, carrying everything from rubber chickens to rocket fuel, lumbered along the tracks, its conductor, a seasoned veteran with a handlebar mustache and a mischievous twinkle in his eye, occasionally checking his watch and wondering if he'd ever get to retire and finally see the world without the constant rumble of steel wheels on iron rails.
The key takeaway is that a freight train sentence is characterized by its length, complexity, and the feeling of weight it carries, just like the train itself.