Do emergency physicians always work on a trauma team?

Usually: In most cases, the surgeon led trauma team features emergency medicine specialists. They add their broader based skills (e.g. Medical management of other diseases) while the surgeon provides technical skills. In some trauma centers, there is no em doctor at the major trauma alerts, however they still play a role in identifying patients with very subtle signs of serious injury. Actually harder!

Yes, in the US: Yes. Emergency physicians work in the ed, which is the first place any trauma patient is taken. In level iii trauma centers and non-trauma centers they may be the only physician that sees them. In level i and ii centers, a trauma surgeon is part of the team as well.

Only at Start: The emergency physicians are there to triage and stabilize trauma patients so they can be moved on for definitive care. This is done outside the ed in the operating rooms, the icu's or elsewhere. Once they leave the ed the emergency doctors do not follow those patients anymore.