A telephone facility and an electrical facility are required in a tunnel. To reduce the effects of EMI and eliminate the need for additional shielding of the cables, where would each facility be place

Placing the two facilities on opposite sides of the tunnel maximizes the distance between the power and communications cables, which is the most effective passive method to reduce electromagnetic coupling. Because inductive and capacitive interference decrease with distance, sufficient separation can bring EMI below harmful thresholds without requiring additional shielding materials. This is the standard approach in tunnel cable routing to protect sensitive telephone circuits from electrical interference.

Placing both facilities on the same side minimizes separation and increases electromagnetic coupling, which worsens EMI rather than reducing it.

Grouping them on the same pathway for maintenance convenience maximizes proximity between power and communications cables, which maximizes EMI coupling and would require more shielding, not less.

Placing one on the top and one on the bottom of the same side still keeps them on the same wall of the tunnel, which is insufficient physical separation to adequately reduce EMI.

Running cables on the exterior of a tunnel is not a standard installation practice and does not address the internal routing and EMI separation requirements inside the tunnel.