In fact, just last year Airbus completed a flight with their A380 (2nd largest passenger jet in the world) using only biofuels.
The US DoD is rolling out big initiatives to address the "post-fuel" era like technologies that convert captured carbon into jet fuel and micro reactors on bases to power these systems. In 10 years or so the same technology will filter down to commercial aviation and everything will be just fine.
I am actually more optimistic this would happen than people giving up on planes tbh.
The largest container ships consume 250 tons of heavy fuel oil per day, they haul up to 15k containers and are at sea for months.
Filling a ship with enough battery power to replace that isn't remotely feasible. Just charging it at port would be an insane undertaking.
It's a truly crazy amount of energy.
With existing technology, a truly global rail system is highly tractable, if you're willing to forgo a transatlantic crossing (either EU <-> NA or Africa <-> SA). Other than that, bridges and tunnels already connect three continents: Europe, Asia, and Africa.
The biggest challenges are the Bering Strait, the Darién Gap, and the South-Asia to Australia crossings.
The Bering Strait is shallow (30--50 m) and narrow (85 km) enough that a conventional tunnel similar to the Chunnel should be viable. It's interesting to note that extant sea routes are already quite close to an Alaska-Siberia land crossing, as the Great Circle from the US West Coast runs along the track of the Aleutian Islands. With trains' greater speed, freight transit times might actually benefit.
The Darién Gap is a swamp, jungle, and mountain barrier to a continuous land crossing between North and South America, between Panama and Columbia. Roughly 100km of this is not traversed by any established roadway. Environmental, political, cultural, and economic concerns have barred creation of a vehicle roadway, but at least technologically the region should be amenable to rail.
The ocean between Indonesia and Australia is, as with the Bering Strait, reasonably shallow, though parts of it are exceedingly seismically active. A mix of bridge and tunnel connections is conceivable and there are actually proposals that have been ... floated ... such as here:
"Beijing to Sydney by Train: The Potential Development of a Singapore, Indonesia & Australia Rail Network" (2015)
<https://www.aseanbriefing.com/news/beijing-to-sydney-by-trai...>
That leaves the Atlantic as the largest present transport route without a ready option.
Conditions are too rough for a floating bridge, and the ocean is too deep for a conventional tunnel. The option of a submerged floating tunnel, proposed as part of Norway's E39 highway, might offer an opportunity for a continuous rail link between both North America and Europe, and possibly South America and Africa (say, Recife to Freetown or Monrovia). Both would be extraordinarily ambitious and would strain extant technology, but are at least theoretically possible.
Transit times would depend on rail speed.
At 320 kph, a 3,200 km (200 mph, 2,000 mile) transatlantic crossing would be a 10 hour journey, ideal for a night train. A 480 kph (300 mph) speed, fastest present tracked rail, would drop that to 6h 40m. At 970 kph (600 mph), roughly jet airliner cruise speed, 3h 20m.
Advantages over air travel should be greater energy efficiency, elimination of turbulence and weather considerations, possibly greater per-passenger space and luggage allowances, and far more continuous departures and arrivals. Disadvantages would be lack of view, technical risks (including catastrophic system failure), and likely longer transit time. I suspect that maximum tunnel speeds will tend to reflect present train systems, which range from 160 -- 300 kph (100 -- 185 mph).
Transoceanic rail crossings have some history at least in the proposal stage: