Reducing cost certainly is a big carrot, but not one that is compatible with safety, no matter what they might claim. Witness Boeing and the Max fiasco where they tried to save money - a fiasco that killed 300 people.
Improved navigation infrastructure and modern equipment with improved accuracy and ease of use removed the need for a separate navigator in the cockpit. Ditto the radio operator, ditto the flight engineer.
But this does not mean that it is sensible to reduce the aircraft cockpit crew to one pilot. Passenger long-haul aircraft don't cross oceans with a single engine.
Engine failure, depressurisation and fire are three major risks that would be extremely compromised if only one pilot was in the cockpit, while the other was sleeping in their bunk.
Major electrical failure would be another - we had an intermittent main generator failure once, that knocked out the A/P and the A/Thr, and it would have been chaotic with only one pilot to hand fly and deal with and solve the rapidly changing problem. QRH and memory drills are based on one pilot flying and another pilot monitoring, and performing emergency QRH tasks.
Trains have single drivers, but they run on tracks, in one dimension, at ground level. Any problem, such as incapacitation, and the train will just apply the brakes and stop. Ditto engine failure. Trains cannot depressurise and even if there is a fire, the train can stop and all the passengers can simply get out.