The earliest disaster to affect the interns of Seattle Grace Hospital arrives in the form of a train crash during the second season. None of the doctors of Seattle Grace are involved in the crash itself, but Meredith becomes emotionally attached to Bonnie and Tom, two passengers on the train who have been skewered together by a metal pole ripping through their torsos.
It's an emotional episode, as Meredith projects her own romantic baggage onto her patients' situation, likening the choice of which patient's life to save to Derek's choice between whether to stay with Meredith or reconcile with his wife, Addison. Still, angsty as it is, the train crash doesn't have much of a lasting impact on anyone at the hospital, with even Meredith moving on pretty quickly.