they might add cars not trains since remember they
are mostly a host on the freight railroad track
they dont own the tracks (expect in the NEC)
even if they wanted to add trains they can only during certain times of the year
they just cant go out and add train "tomorrow" for example

Amtrak would love to increase its route structure, but can only do so with financial support, most of which comes from the US Congress. The current President isn't a fan of passenger trains, and proposes only minimal budgets for Amtrak--not enough to add any routes. Congress usually increases funding somewhat, but again not enough for new routes.

Individual states can pay Amtrak to run more trains in their state, but only a handful of states have done so.

If you're interested in learning more about this issue, visit the website of the National Association of Railroad Passengers, which aggresively supports an expansion of Amtrak's routes and service. Their website is:
http://www.narprail.org

Ridership is certainly increasing. Number of trains won't, much, because Amtrak doesn't have spare cars lying around.

They have a great pile of derailment-damaged cars they don't have the money to fix (cars are made of stainless steel, so they're pretty much indestructible except for derailments). Some states like Illinois and California subsidize Amtrak service in their state, and sometimes one of the states will pay to fix up some of the cars, for service in their state.