A Victorian rail link and wartime supply

How early could the proposed rail link be built, and what would it change in the world wars?

A Victorian project is considered possible despite its cost and danger; it could simplify Western Front supply and later serve as a withdrawal route before being destroyed.

The proposed Victorian rail link appears to be a Channel Tunnel, although the surviving question does not name it. Brunel is apparently the engineer intended in the reply, which stresses difficult construction, high cost, and loss of life. Broad gauge is suggested. In World War I the link would make supplying the Western Front much easier. In World War II the army could fall back toward it, although the link would probably be blown up.