1) The only artillery that can shoot over troops, it mean without line of sight(LOS) is the Trebuchet in Bretonnian list. All the other artillery need LOS to fire.
2) In my opinion, Steam tank has armour of 3+ and count as defended. For example Cannons, denies armour save and enemies in fortified count as defended. Enemies in defended count as open.
So in my opinion,
The rules: if a cannon shoot a steam tank, count as open and denies the armour roll.
The fluff: Steam tanks are powerful but not cannon-proof.