Mass-casualty training at Keesler Air Force Base two months ago gave Biloxi firefighters and police the chance to practice responding to a train crash. The first-responders couldn’t know they would soon be responding to an actual train-charter bus crash that killed four people and critically injured eight of the 49 passengers. The bus driver made the total bus load 50 people. An incident command team and dozens of firefighters and medics tended to the injured Tuesday afternoon at the Main Street crossing, sending 41 people to five hospitals from Gulfport to Pascagoula.