The LNG tankers are more dangerous empty (fumes) than full (when it’s full of what is essentially gas jelly).

IIRC (and not sure if it’s still the case) they used to have to time the LNG ships so that the tide was just right; if it was too high it would not clear the Tobin, and if it was too low there was a possibility of the ships crashing the harbor tunnels.

Essentially this is more efficient. But also the rail infrastructure to Moran Terminal in Charlestown, which is aka Boston Autoport, is in poor shape and not been used in many years. It also snakes it’s way through a highly congested area (both streets and rail) and would be a very slow and disruptive way to ship that many cars, using various freight railroads, routes, etc.