No. The rule is much squishier than that.
The fact is that Amtrak will not start any new route without guaranteed earmarked outside funding -- unless it's profitable-before-overhead like Lynchburg is -- because Amtrak has no reason to expect increased general-fund funding, and needs to preserve that funding for the existing service.
Amtrak is required to use the PRIIA rules for cost -- which include lots of overhead allocation -- for routes under 750 miles. For such a route, Amtrak will only start the route if the state covers the PRIIA-specified allocation.
For a route of more than 750 miles, Amtrak will start the route if the states cover the *net avoidable costs*. They made this quite clear with regard to the Gulf Coast situation. If the Federal government provided an earmark for the route (only usable for that route) which covered the net avoidable costs, I am sure Amtrak would also start the route. This is Amtrak policy, this is not the law, but it is a policy which makes sense given all the other demands Amtrak has on its limited general-fund money.
If a >750 mile route would actually provide profit before overhead, Amtrak might start it without subsidy. The only possibilities for that which I see in the near future are a daily Cardinal, and through cars from the Pennsylvanian to the Capitol Limited.