You may be surprised to know that this is actually something people have thought about; turns out you would need extremely low utilization of the train, plus a very long journey at each end by car, plus one of the more inefficient train routes, for it to lose to the car or the plane. High speed trains are generally the least efficient in CO2/passenger km, so if you’re looking to put this together then I would recommend France’s excellent analysis of their carbon output on their routes.
As one example, the Paris-Geneva TGV Lyria is 3.5 kg co2e per person per trip on an average utilization train. [0] Driving is 149 kg co2e per vehicle; average occupancy is 1.12 to 1.6 people per car but let’s be generous to your idea and say we have a full average Swiss family of 2.3 people along for the ride. So driving is 64.7 kg co2e per person. It’s a 5.5 hour drive, so let’s be very favorable to the car again and say they have a 1 hour taxi ride at each destination. You’re now at a total of 23.2kg taxi + 3.5kg train = 26.7kg total, or half that of driving; as you can see even a 10x capacity reduction in the train would still make the train more efficient. It’s remarkably hard to beat a train with anything other than a bus, and you’ll need a fairly efficient bus and a fairly inefficient train for that.
As one example, the Paris-Geneva TGV Lyria is 3.5 kg co2e per person per trip on an average utilization train. [0] Driving is 149 kg co2e per vehicle; average occupancy is 1.12 to 1.6 people per car but let’s be generous to your idea and say we have a full average Swiss family of 2.3 people along for the ride. So driving is 64.7 kg co2e per person. It’s a 5.5 hour drive, so let’s be very favorable to the car again and say they have a 1 hour taxi ride at each destination. You’re now at a total of 23.2kg taxi + 3.5kg train = 26.7kg total, or half that of driving; as you can see even a 10x capacity reduction in the train would still make the train more efficient. It’s remarkably hard to beat a train with anything other than a bus, and you’ll need a fairly efficient bus and a fairly inefficient train for that.
[0] https://www.tgv-lyria.com/sites/default/files/inline-files/t...