Cwatters,
Some places activate the "Major Emergency Plan" when a serious incident that may involve injury to or death of more than a certain number of people (I remember 12 from a seminar a few years back ). Once the M.E.P. is actioned, by a senior official (Perhaps Local Authority Mgt, Senior Police Officer ), then a "set piece" response swings into action. Emergency services position to the scene or likely scene, police cordon off areas, Ambulances Fire Appliances dispatched, auxiallary fire and ambulances called to duty and to answer your question, Hospitals brought to a status where they can readily recieve and cater for multiple casualties, this may involve cancelling some routine services, preparing facilities, mobilising staff and standby staff for "incomig".
I am a Pilot, not a medic, but had the chance to attend a briefing on such about 10 years back, in Ireland. Some may think such a strategy "over the top". But in reality, it is used to deal with serious incidents every day, such as serious traffic accidents, industrial accidents, terrorist threats and I remember Aircraft Emergencies being on the list too, because of the sheer number of possible casualties involved.
Of course in modern day political society, if the MEP is actioned and no crash/injuries occur, then many will cry "overkill". But if an aircraft full of pax did crash, and the MEP had not been actioned, then the same people would be screaming blue murder, looking for the guts of the person who did not action it.