It is time but also energy. At 1000kph a train (at sea level) has massively more drag than an a380 at altitude. Even on electric, these things may have a greater carbon and financial cost than proponants want to admit.
An electric train can have near-zero carbon footprint, because electricity can be generated from nuclear fuel, sunlight, wind.
An electric airliner is still unattainable. Synthesizing jet fuel using electricity is of course possible, but must be massively less efficient than using that electricity directly in electric motors with efficiency > 0.95 and recuperative braking.
The carbon footprint of building the railway, likely with massive amounts of concrete to use to support the rails, and the steel for the rails (to say nothing of maglev infrastructure) — that's going to be huge. Building a pair of airports should be much less expensive.
