The most important thing you should do is first seek medical attention if you need to, and absolutely get an incident report from the MBTA. That is important.
Document the scene with photos if possible and contact information from witnesses that saw the accident. File an incident report, again with the MBTA.
Keep the detailed records of your medical treatment and expenses and also remember that the claim against the MBTA have reduced notice requirements. So, absolutely contact an attorney as soon as possible.