Amtrak is offering a free Acela Express or Metroliner roundtrip when you take two roundtrips between select cities through May 31.

They’re also offering double Amtrak Guest Rewards points for tickets purchased with a Mastercard.

Registration is required.

Eligible city pairs are:

Boston – Stamford, Boston – New York, Boston – Newark, Boston – Metropark, Boston – Washington, Boston – Philadelphia, Route 128 – Stamford, Route 128 – New York, Route 128 – Newark, Route 128 – Metropark, Route 128 – Washington, Route 128 – Philadelphia, Stamford – Washington, New York – Washington, Newark – Washington, Metropark – Washington.

You can earn up to 8 free roundtrip certificates valid for travel between June 15, 2005 and December 15, 2005.