Southern Railway posted record fourth-quarter and 1981 net income despite a 7.4 percent decline in the number of freight cars it loaded, the Washington-based railroad reported today.

Southern President Harold H. Hall, meeting with securities analysts here, said the railway earned $52.1 million ($3.32 a share) in the last three months of 1981 on revenues of $450.8 million. That compares with net income of $44.1 million ($2.82) and revenues of $429.6 million for the final quarter of 1980.

For the year, Southern earned $212.1 million ($13.55) on record revenues of $1.79 billion compared with net income $180.9 million ($11.57) and revenues of $1.638 billion in 1980. Earnings were up 17.2 percent on the year and revenues rose 9.3 percent.

Hall said the increase in earnings came because of higher rates for the merchandise Southern transports as well as economies of operation. He noted that while carloadings were down 7.4 percent, revenue ton miles--the number of miles a railroad carries a ton of freight--declined only 2.6 percent.

Hall said this reflected an increase in the average load carried by an individual freight car and an increase in the average distance each freight car travelled.

Southern and the Norfolk and Western plan to merge later this year if the Interstate Commerce Commission approves. N&W reported record income last year, too, mostly because of continued strong demand for coal, the principal commodity N&W hauls. Southern and N&W are consistently the nation's most profitable and efficiently managed railroads.

The proposed merger is partly a defensive move to counter last year's combination of the Seaboard Coast Line and the Chessie system into the CSX system, which operates principally in the areas that N&W and Southern serve. The CSX, which can offer more direct service from the South to the Midwest and vice versa than can either N&W or Southern independently, has taken business from N&W and Southern.

"We'll get that business back and more," after the merger, Hall said in an interview after today's meeting.

Hall said that while Southern is pleased with 1981's results, the current recession is worrisome. Many of the products that Southern hauls, from automobile parts to lumber and wood, are adversely affected by the declining economy. Southern also has been hurt in recent weeks by the snow and extreme cold in many of its service areas.

Coal, he said, remains a bright spot. "I wish we could say the same about the other commodities" the Southern transports. He said he anticipates a 6 percent increase in the amount of coal carried this year.

Separately, other Washington-area companies reported earnings:

Union Trust Bancorp of Maryland and two Washington firms-- D.C. National and Security National banks--have reported record profitability from the banking business in 1981.

Baltimore-based Union Trust, holding company for the $1 billion-deposit Union Trust Co., said operating profits soared 78 percent to $12.8 million ($5.23) from $7.2 million ($2.94). Net income, including gains or losses from selling securities, jumped 43 percent to $10.4 million ($4.26) from $7.3 million ($2.97).

A key to the regional banking firm's gains was a 23 percent rise in interest and fees on loans, as interest rates hit record levels. Braced with the record performance, Union Trust directors have boosted the dividend on common stock by a nickel a share to 42 1/2 cents quarterly. Fourth-quarter profits from operations rose 101 percent to $3.6 million ($1.47) compared with $1.8 million (73 cents).

Assets rose 11 percent in 1981 to $1.5 billion while deposits at 79 branches throughout the state rose 14 percent to $1.08 billion.

D.C. National Bank said operating profits rose 16 percent last year to a record $2.2 million ($10.39) from $1.89 ($9.22) the previous year. Net income was $2.06 million, for a return on shareholders' equity (stock investment) of 22.9 percent.

The city's seventh-largest bank, D.C. National recently won regulatory approval to form a holding company, D.C. National Bancorp Inc. Assets rose last year to $157 million from $138 million at the end of 1980, Chairman John Safer said.

Security National, which recently acquired Washington Bank N.A. in D.C., said operating profits rose 20 percent last year to $2.3 million ($5.77) compared with $1.9 million ($4.80) a year earlier. Net income was $1.85 million ($4.62) compared with $1.84 million ($4.61) in 1980.

Fourth-quarter profits declined, however, to $422,356 ($1.06) from $537,821 ($1.34) a year earlier. President Thomas Schaefer said the lower earnings in the recent quarter were due "to some extent to nonrecurring expenses." He said assets rose 14 percent to $132 million.