Wreckage is left behind when a steam unit is destroyed. Wreckage will block all ground movement through the square its on and, if it is not removed, lasts 6 ticks except in Wendigo Attack where wreckage disappears faster. The only way to remove wreckage is by using the Tumbleweed Wind card.
If a Dirigible is destroyed and crashes into an enemy unit, it will not leave a wreckage. If there are no enemy units in range to crash into, it will leave a wreckage wherever it lands.
It is possible to tell from the wreckage what type of unit it was. The wreckage also retains the colour of the player the unit belonged to. This has no influence on gameplay.