In the City of Worcester, there are two ambulance companies that primarily back up Worcester EMS...Vitals and Medstar. Neither Eascare or AMR provide backup service unless the city has really exploded. Worcester EMS staffs (4) Medic trucks full time and (1) impact truck.
If you are looking for good experience and hoping to get some 911 time also, consider MedStar. They not only back up Worcester, but also hold the contracts for the City of Fitchburg and Leominster.
Hope this helps,
Stay Safe Out There