I think your estimate is quite off here: Train connections from Berlin to Paris take about 8.5 hours. Flights take 1 hour and 50 minutes plus the overhead time for traveling to and from the airport, security and taxiing. Just let's suppose that this will be 2.5 hours in total, which would be very fast. So we are comparing 8.5 hours with 4.5 hours. So, the factor is less than two rather than the four to five you suggested.