How Deep Is the Train Tunnel? The Question Contains a False Premise

There is no single "train tunnel" with a universal depth—and certainly none that "passes through the Earth's crust" as the question implies. Train tunnels vary enormously by geography and engineering. Here's the real picture:

Depth Ranges by Tunnel Type

• Urban metro tunnels: 10–50 m below surface (cut-and-cover or shallow bore). Deep stations: 80–100 m (e.g., Kyiv Arsenalna 105 m, Pyongyang 100+ m).
• Mountain base tunnels: 500–2,500 m below mountain peaks (not surface). Gotthard Base Tunnel (Switzerland): 2,450 m max rock cover. Lötschberg: 1,800 m. Brenner Base (U/C): ~1,600 m.
• Undersea tunnels: 50–250 m below seabed. Channel Tunnel: 75 m max below seabed (40–75 m water above). Seikan (Japan): 140 m below seabed (100 m water). Ryfast (Norway): 292 m below sea level (deepest road/rail tunnel).

What "Through the Crust" Would Mean

Earth's crust is 5–70 km thick. The deepest human borehole (Kola Superdeep, Russia) reached 12.3 km—not a train tunnel. Temperatures at 10 km exceed 180°C; rock behaves plastically. No engineering exists for transit at that depth.

So the Answer Is: It Depends

Specify the tunnel. The deepest rail tunnel by rock cover is Gotthard Base (2.45 km). The deepest by elevation below sea level is Ryfast (–292 m). The longest is Gotthard (57 km). There is no universal depth.