* Trains ultimately require close to 1 staff member per 4 passengers, much more than a budget airline.
* Unions and legacy salaries further add to those personnel costs. The average Amtrak employee ($75K/yr) nearly makes as much as the average commercial pilot ($77K/yr).
* Since train journeys take longer, the labor cost per trip is considerably higher, especially for long haul routes. When you're doing a long trip like Chicago to Los Angeles, a train takes a lot longer than a plane, so you're paying for yet more labor for that trip. 44 hours of labor for the train versus 4 hours for the plane.
There's a good video that breaks down the costs that make up your train ticket: https://www.youtube.com/watch?v=fwjwePe-HmA&t=29s