War Engines do not "push units" when they barge, the units move out of the way and are placed as near to where they started as possible touching the engaging WEs base. With this in mind the only way it could happen is if the WE managed to travel through the terrain itself. As it is not allowed to enter most impassable terrain itself it cannot do this (jungle and rivers only exception I can find for WE and non WEs being different) .
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3.3.1 Charge Moves
When a war engine charges it is allowed to ‘barge’ any non war engine units belonging to the target formation out of the way and carry on with its charge move. Move the war engine as far as desired, and then place any units that were barged out of the way as close as you can to their starting point, while still touching the base of the war engine that so rudely pushed them aside. The maximum number of units a war engine can barge aside in this manner is two per point of its starting damage capacity. Note that war engines may not barge other war engines out of the way.
Also from terrain 1.8
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The terrain is impassable to the unit so it cannot move
through the terrain under any circumstances.