First of all, without being able to ride on the coattails of Amtrak's right of access via proxy, I cannot foresee any private railroad accepting a third party operator on it's lines.

Secondly, these services still need to be able to cover the variable costs as well as a delegated percentage of the fixed costs. Ticket sales alone won't accomplish this, even if the service becomes that special something people fall all over themselves just to experience. Thus, these third party passenger services will have to be able to offer some types of freight services as well.

Finally, as was pointed out in previous threads, the frequency of the service would need to be such that a traveler does not have to wait up until 3 in the morning just to utilize the service from their particular area of residency. Long distance trains would need to be sectionalized to where the service is available in 12 hour frequency rather than the current 24 hour (or three days a week) schedules. This would allow all logical stops to be utilized in nominal daylight hours.

Again, there's a lot of critical minutia that needs to be addressed before the idea of viable passenger train service can be realized.