Note:Report is being filed as a precaution. A fire developed in the exhaust stack of the 3744 Locomotive, that lasted 15 mins. and was blown out by throttling engine up. Approximately 10 min. period of time past without a fire. Then a fire started again in the exhaust stack area and was extinguished by local fire dept. using 1 extinguisher. The second fire burned for 15 mins.