Isn't there a plan to have one of the four tubes cross under another tube? If so, why would this be the case?
In the B&P tunnel replacement? Not that I am aware of. What four tubes?
Alternatives 3 and 11 have 4 bored single track tunnels. They dropped the concept of a larger diameter bored tunnel for 2 tracks because of constraints of the proposed route and depth profile. And, yes, the diagrams in the Alternative report show one tunnel ducking under the others in the various options.
Quoting from the executive summary in the report:
Alternative 3 and Alternative 11 would replace the B&P Tunnel in a new location. Consideration of a double track tunnel was eliminated from both alternatives because of its much larger tunnel diameter (about 50 percent larger compared to a single‐track tunnel), the tight profile constraints posed by the design criteria, portal elevations, and intermediate underground obstructions. The resulting configuration is using four single‐track tunnels for all alignment options for Alternatives 3 and 11. Horizontal excavation (boring) is proposed for these alternatives to minimize surface impacts. Four tracks in four separate bores of equal size would support train capacity requirements, service flexibility for conflict‐free operations, design within physical constraints, and constructability. The tunnel vertical clearances for both Alternative 3 and Alternative 11 would also accommodate double stack container freight.
Alternative 3 and Alternative 11 each incorporate a subterranean grade‐separated track crossing or “duck under” approach to aligning the four individual tunnel bores to minimize conflicts between turning trains and increase operational efficiency, while correctly aligning tracks with those being planned at Penn Station. Each option includes ventilation plants at permanent portals and at an intermediate location along the tunnel, and emergency egresses. Outside approaches to portals for each would consist of open trench transitioning to cut‐and‐cover to the portal entrance. Each provides universal interlocking to the NEC mainline and avoids the Metro Subway tunnel while servicing the West Baltimore MARC Station. All of these options would relocate a pier of the CSX Baltimore & Ohio Bridge in Jones Falls Valley. All Alternative 3 and Alternative 11 options also consider disposition of the existing tunnel.
The price tag of the B&P tunnel replacement is going up with the decision to expand the scope of the project to 4 new tracks in 4 bored tunnels. The price range for Alternative 3 Options A, B, C is from $3.7 billion to $4.2 billion. Alt 3 Option A has the smallest impact on the taking of property and land and the least expensive at $3.7 billion at the tradeoff of less in travel time savings for the Acela, Regionals, and MARC.