U.S. Army Reserve firefighters with the 468th Engineer Detachment, Danvers, Massachusetts, assess the situation at a train derailment exercise, part of Guardian Response 17 at Muscatatuck Urban Training Center, Butlerville, Indiana, May 7, 2017. Nearly 4,100 Soldiers from across the country are participating in Guardian Response 17, a multi-component training exercise to validate U.S. Army units’ ability to support the Defense Support of Civil Authorities (DSCA) in the event of a Chemical, Biological, Radiological, and Nuclear (CBRN) catastrophe.