The customary units for carbon emissions is tons of CO2. In talking about passenger aircraft for comparison purposes, it might make sense to compare carbon emissions per passenger mile. In this case I think a modern aircraft at full load compares favorably to a passenger car with 1 or two passengers. A bus full of people will do significantly better, but a bus with three passengers does worse. For Electrified rail, the efficiency is higher again, but the calculation now has to talk about the efficiency of the fossil fuel component of the generating mix applying in the country of travel.