As I said earlier though, the problem with batteries is they're a PITA to extinguish if they catch fire and you don't want to have that problem deep underground

As I said earlier though, the problem with batteries is they're a PITA to extinguish if they catch fire and you don't want to have that problem deep underground unless you plan to flood entire parts of the tunnel (which brings its own problems not even considering cleanup itself). As any emergency worker will tell you, the problem with batteries compared to ICE engines is that if the battery is on fire it doesn't just stop once you extinguish it. You need to keep at it until it has cooled down sufficiently to prevent spontaneous reignition and that takes a while. Plus the entire time it burns it generates large amounts of toxic fumes. Compare that to an electric line that just needs to transmit electricity and just stops if you shut it off. Yeah, ozone accumulation is a problem but so is heat so you'll need some venting anyway (plus you probably want the air in the tunnels to be breathable because we're humans after all).