Studies have considered many alternative routes for a new tunnel. The 2014
report linked below eliminated for further consideration all but routes 2,
3, and 11. Route 2, which involves rehabbing the existing tunnel, is the
only one that can preserve a connection with the ex-WM at Fulton Junction.
Rehabbing this tunnel as well as the B&O's Howard Street Tunnel is likely
less expensive than building any new route. As a temporary bypass while the
rehabbing effort is underway one tube of either the I-895 Harbor Tunnel Thruway
or I-95 Fort McHenry Tunnel could be adapted for trains. Extensive rail
infrastructure already exists near the portals of both tunnels.
During December 2016 the Federal Railroad Administration announced 3B,
a tweaked version of alternate 3, as its recommended route.
Alternative number 5 along US 40 was too quickly dismissed because it does not
serve Penn Station. Disused ex-Nothern Central trackage already exists parallel
to I-83 that could easily be rejuvinated as a short spur to Penn Station, though
trains lacking an engine at both ends would need to be turned to exit.
2014 B&P Report (huge 100m PDF),
FRA 2016 decision