If it is profitable, why involve the government? And why expand it to cover new areas across the country? If it was/is profitable, there should be investment capital galore just waiting to get in on the potential profit.
The sad fact is the reason it is not expanding via private investment is that it is probably not really profitable, but the feds cook the books so they can tout their success.
These are Electric Trains, and the wiring and transmission lines have to be somehow squirreled in to the existing cities, suburbs, and rural areas. Electricity is neither cheap nor "green". Somebody's burning coal somewhere so those trains can run "clean."
In claiming that the service is "profitable", Amtrak doesn't charge any of the cost of the equipment, track or other capital facilities. In government accounting, those are paid for by the tooth fairy.