You seem to have focused your 'tunnel vision' on the costs of boring that thing alone. Sort of CAPEX, so to speak. But what about less OPEX later, because of m

You seem to have focused your 'tunnel vision' on the costs of boring that thing alone. Sort of CAPEX, so to speak. But what about less OPEX later, because of more flexibility, less maintenance burden, when other demands arise or conditions change? If the tunnel is wide enough, two tracks can be built side by side, and another level of two tracks on top of it. That makes room for a total of four (potential) tracks, with the possibility to switch between them not only horizontally, but also vertically via ramps (inside that tunnel profile). It also enables single platform stations for one track on one or two of the two possible levels in such a tunnel. Without any further 'big digs', or blocking the line altogether.