Apparently, there were liability concerns so the owners of Heritage Railroad weren't able to renew their Operating Agreement. Here's a quote from their homepage (in case it goes down):
This makes two excursion railroads shut down within the past month. However, the SARM will continue being able to operate an "as needed" freight service on the line.Welcome to the Southern Appalachia Railway Museum!
SARM Farewell to the Secret City Excursion Special Last Trips- Online Sales here
It is with great regret that we have to announce the suspension of the Secret City Scenic excursion operation. We received notification earlier today from the owners of Heritage Railroad, that they would be unable to renew our Operating Agreement. The reason given was liability concerns.
In nearly two decades of service SARM is most proud of the thousands of volunteer hours worked and tens of thousands of passengers transported safely.
The museum's extensive equipment collection is safe and we will be working on other opportunities in the future. We would like to thank the Department of Energy, CROET, Energy Solutions, the Oak Ridge Heritage Rail Authority, Walden's Ridge Railroad and the Oak Ridge community for their years of support in this endeavor.
We will be refunding all pre-paid reservations.