I really like the concept of a train moving through objectives!
My main thought on balance is the train seems extremely resilient. This might make it quite challenging for the Ambusher to win within 4 rounds if the Engineer maintains speed. The 'Don't stand on its way' rule's 20-wound threshold for speed reduction also feels very high. It could lean slightly Engineer-favored, but playtesting would confirm!
Great work on a creative mission!