In the beginning of act two for red falls you will go to a building where you'll take a train. Before you can leave you'll have to activate the power. Kill the zombie. All you'll have to do is flip the switch in the back, then flip the switch on the control panal, then just flip the switch in the back one more time to give power to the train. But before you get into the train check the body of the zombie in the control room. You will find fingerprint liquid. Use this liquid on the bomb and it will show you the three digit number. This number will defuse the bomb.