CSX Corp., of Richmond, yesterday reported record earnings of $104.8 million ($2.54 a share) for the three months ended Sept. 30, an 87 percent increase over the same period last year. Corporate officials attributed the sharp increase to "exceptionally strong domestic and export coal markets" as well as tight expense controls. For the same period last year, CSX reported earnings of $55.9 million ($1.41).

Earnings for the first nine months of 1981, also a record, were $229.6 million ($5.58), 23 percent above the company's earnings of $186.9 million ($4.79) in the same 1980 period. Both major CSX rail units, Chessie Systems Railroad and Family Lines Rail System, set all-time monthly coal traffic and tonnage records in the third quarter.

CSX officials expressed optimism that corporate earnings for 1981 would exceed the record of $281.6 million ($7.13) set last year. "Our optimism results from the dramatic increase in third-quarter earnings that have offset the earnings impact of the 80-day coal miners' strike," they said.

Consolidated revenue for the third quarter was $1.4 billion, up 22 percent from the 1980 figure of $1.2 billion. Transportation revenue was $1.3 billion, up 23 percent from $1.1 billion in the third quarter of 1980. Nontransportation revenue was $93.8 million, slightly above the third quarter of 1980, reflecting increased earnings from natural resource activities and the continued strong performance of The Greenbrier resort hotel and Florida Publishing Co., which are having excellent years, officials said.

Baltimore Gas and Electric Co. reported earnings of $94.39 million for the eight months ended Aug. 31 ($2.85 a share), compared with earnings of $78.28 million ($2.44) during the same period of 1980. For the 12 months ened Aug. 31, BG&E had earnings of $133.42 million ($4.05), compared with $100.95 million ($3.16) during the 12 months ended Aug. 31, 1980.