Chessie System Inc., which operates one of the nation's largest railroad networks, yesterday reported record profits and sales last year despite a dip in fourth-quarter operations that reflected national economic pause.

Chairman Hays T. Watkins reported that for the first time in history - a span that covers 150 years for his firm - annual profits topped $100 million in 1976. Net income was $102 million ($5.38 a share) compared with $91 million ($4.91) the previous year.

Rail revenues totaled $1.41 billion compared with $1.32 billion in 1975. Chessie owns the Chesapeake & Ohio, Baltimore & Ohio and Western Maryland railroads. The B&O inaugurated the nation's rail industry when it was chartered on Feb. 28, 1827.

In fourth quarter alone, Chessie profits fell to $37 million ($1.94 a share) from $40 million ($2.16) a year earlier, although revenues rose to $364 million from $340 million.

Watkins said yesterday that the economy remained dormant in the normally strong fourth period and freight carloading volume slackened.

He forecast "sustained moderate growth" in business this year and cited estimates that steel shipments could rise 10 per cent and coal putput is expected to rise more than 5 per cent.

Cheesie lines are the biggest carriers of coal in the nation and Watkins said he expects coal traffic to increase 40 per cent in the next five years from mines now being built or enlarged alongside Chessie tracks.

In 1976, Chessie carried 95 million tons of coal, of which 78 million tons came from adjacent mines. More than 100 new or expanded mines currently are under construction.

Loadings of vehicles and auto products rose 15 per cent last year despite the Ford Motor Co. strike and movements of chemical products and steel and other metals also were up slightly. Shipments of food, grain, lumber, paper and minerals were close to 1975 levels.

Watkins said Chessie will spend $160 million on capital improvements this year, of which $110 million will be use to buy 3,800 freight cars and 35 locomotives. Some $50 million will be spent for physical additions and improvements following an outlay last year of $202 million on maintenance of roadway and structures.

Although long-range economic prospects are "heartening," and uncertainty is Consolidated Rail Corp., the new Northeast railroad set up by the federal government to supplant bankrupt lines and now a Chessie competitor, Watkins stated.

"We intend to compete vigorously against it for a large transportation market" while cooperating on joint use of some facilities, Watkins added.