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But if we assume that train is a light express train, it always comes through the tunnel at relativistic speed, let's say train speed = 0.8c, relative to the observing engineer.

So the civil engineer who built the tunnel, watches the train gets shorter due to special relativity (Train length relative to the standing engineer is 5m).

Does it make sense to say: since the train crosses the tunnel at relativistic speed close to the speed of light, so the engineer can build a shorter tunnel (length 5m) and tunnel can still encompasses the full length of the train?!? Engineer has saved money on the lower cost shorter tunnel.!