Airline deregulation allowed prices of airfares to drop. If, by one means or other, airfares are kept HIGH, trains become more popular for medium-long trips, especially if the railroads work to increase speed and/or comfort. Getting on a train at 5 or 6 pm, and waking up at your destination, means that you spend little waking time traveling.
If jet airplanes are less trusted, with more fiery crashes, that might keep people more interested in staying on the ground. (Perhaps there's more crashes in several countries due to metal issues like the Comet, and some popular people die.)
And, of course, politics. If the railroad companies have more political clout, they can buy a few influential congresscritters to help hamstring the jet industry...