Thomas Cullen III, Sqd. 41 STATEN ISLAND, N.Y. — Every morning when Thomas Cullen IV came downstairs, there was a new adventure waiting for him. After the 2-year-old fell asleep each night, his father would quietly construct a new train track design in the living room. Thomas P. Cullen III spent his days fighting fires and thinking about what he could create next for his son. “My husband would get so excited about building the track,” said his wife, the former Susan Quinn. “He wanted Tom to be excited when he came downstairs every morning. He spent his day thinking about how he would design it. He would always say, ‘I can’t wait for Thomas to go to bed. I have a good idea for a new track.’ ” Father and son could spend hours playing Thomas the Tank Engine together, but the toddler’s best playmate isn’t there to build tracks for him anymore. Mr. Cullen, 31, is missing in the attack on the World Trade Center. On the morning of Sept. 11, Mr. Cullen left a message on his answering machine, wishing his son good luck on his first day of preschool. It was 8:23 a.m. Twenty-five minutes later, the first jetliner slammed into Tower 1. Mr. Cullen was in the middle of a 24-hour shift with Squad 41 in the Bronx. “He left a message for my son,” Mrs. Cullen said. “That was the last we heard from him, and that was on the answering machine.” Mr. Cullen was last seen in Tower 2. “He loved the Fire Department,” Mrs. Cullen said. “If he didn’t get into the Fire Department, he was going to go to law school, but the Fire Department was always what he wanted to do.” After graduating from the Fire Academy in 1996, Mr. Cullen was assigned to Engine Co. 290 in Brooklyn. He rotated to Ladder Co. 83 in Westerleigh, and joined Squad 41 two years ago. Prior to joining the Fire Department, he was a lifeguard instructor for the city Parks Department. Mr. Cullen was passionate about two things — his family and fighting fires. He did everything with his college sweetheart, even accompanying her on trips to the grocery store. “We did everything together, and that is why the void is so big,” Mrs. Cullen said. “I can’t even go to the grocery store.” But what she will miss most about the man she calls her best friend is watching him with their son. “He was a wonderful father,” she said. “Our son was the be-all and end-all for him.” Born in Brooklyn, Mr. Cullen moved to Silver Lake in 1996 and to West Brighton in 1997. He graduated from the Loyola School in Manhattan in 1988, and earned a bachelor of arts degree in political science in 1992 from Fordham University in the Bronx. A sports enthusiast, he played quarterback on the Fire Department football team and basketball on the firehouse team. He was a certified scuba diver, and loved the clear waters of the Florida Keys. On a trip to Cabo San Lucas, Mexico, in December, he explored the dazzling marine life of the Baja peninsula. In addition to his wife of five years, Susan, and his son, Thomas, surviving are his parents, Rita and Thomas Jr.; his 18-year-old brother, Sean, and his sister, Rita Cullen. A memorial service was held Oct. 5.