> Perhaps trains beat road transport efficiency to a similar degree.

Not just efficiency but you can use electric trains if your tracks are electrified. Add into that electricity production system that is mostly renewable+nuclear (the Nordics for example) and you get very very low emissions.

They do, trains are BAFFLINGLY fuel efficient in terms of pounds of cargo. Once they get up to speed, trains can move one ton of cargo about 480 mile per gallon, vs 130 with trucks

This is why European cities had so many canals back in the day.

classic volume grows faster than surface area.

also, fuel is a huge cost (maybe even the main cost), and drag has a >linear relationship with speed- so the ships will slow down based on fuel prices.