A few snaps of freight cars I have built (so far)
These are a chemical hopper car and a railbox (next load, any road) logoed box car.
The hopper is made of wall sections, with rounded ends from my 6441 divers base. You can see the hatches of the boxcar in the overhead shot on the passenger train page.
The box car has a giant X logo embedded in the wall on each side. The door does not work, but it is stepped back 1/2 stud with the 1x2 center stud plates trick that Kirk Unit taught me.
This is a depressed center flatcar. The load is 8 studs wide, and 9 bricks high. The car has 8 axles. Each group of two axles is independent and pivots on its own beam. The beams then pivot about where the diagonal braces (technic axles)join. Because I did not have long enough technic plates, one bogie pivot is 1/3 brick higher than the other.
The diagonal braces are for show, they slide through a cross axle connector at the bottom. Since they are connected at the top to a 1x1 with side stud, they cannot stand the strain if you put 1/2 stop bushings below the cross axle connectors. You can see that the braces actually are needed as the car sags in the middle from the weight of the oversize load.