Depending on the country, there are significant requirements on the "box" regarding how much it compresses end-to-end in case of accidents. "Crush" requirements

Depending on the country, there are significant requirements on the "box" regarding how much it compresses end-to-end in case of accidents. "Crush" requirements are in the range of hundreds of thousands of pounds of force, IIRC. They are especially high for passenger cars operating on the same tracks as freight.