An alert Waltham Police Officer spotted a house fire about 5:30am this morning at 261 Derby Street in Newton right on the city line. Officer Tony Schocolne notified dispatch who in turn called Newton Fire. The officer successfully evacuated the residents just as propane tanks under the front porch exploded. Tow cats were found alive by Firefighters about 7am, 90 minutes into the fire, as they began to re-initiate and interior attack. One bystander was taken to the hospital for evaluation.