Southern Railway Co. reported record third-quarter earnings yesterday, largely because of general rate increases and cost-control measures and a strong rebound of coal traffic after the 73-day miners' strike this spring.

Net income for the quarter was $48.3 million ($3.08 a share) compared with $36.2 million ($2.31) for the same period in 1980. Total revenues were $484.0 million compared with $409.1 million.

For the nine months ended Sept. 30, net income was $160.0 million ($10.23) compared with $136.8 million ($8.76) in 1980, and revenues were $1.4 billion compared to $1.3 billion.

Car loadings of coal in the third quarter increased 17.6 percent compared with the same quarter last year. Nine-month coal revenues increased 17.7 percent compared with the same quarter in 1980.

"We are also pleased that quarterly car loadings and revenue-ton-miles were up for both chemicals and allied products and pulp and paper, Southern's second and third principal revenue producing commodities behind coal," said Southern's president, Harold H. Hall.

All of Southern's 12 revenue-producing commodities except for farm products and aggregates including other nonmetallic minerals, contributed to the record revenues, the company said. For the nine months, all of the commodity groups except aggregates were profitable.

Easco Corp., a Baltimore-based manufacturer of mechanics' hand tools and aluminum products, reported an increase in third-quarter earnings from $1.7 million (48 cents a share) in 1980's similar period to $3.8 million ($1.11) this year. Net sales were $113 million compared with $93.4 million in 1980.

For the nine months ended Sept. 30, net income before capital gains was $9.1 million ($2.71) compared with $1.4 million (27 cents) for 1980. Net sales were $316 million compared with $244 million.

The nine-month results last year were severely depressed because of an operating loss during the second quarter of $1.2 million. The loss was attributed to significantly lower hand-tool shipments and the sluggish economy. However, the company said hand-tool shipments have improved and the company's aluminum products group showed a strong performance this year.

Union Trust Bancorp, parent of the Union Trust Co. of Maryland, announced an increase in third-quarter earnings before security losses of $3.39 million ($1.38 a share) from $2.07 million (85 cents) last year. For the nine months ended Sept. 30, net income before security losses was $9.2 million ($3.75) compared with $5.4 million ($2.21) for 1980. On Sept. 30 assets were $1.4 billion, deposits were $1 billion and loans were $850.8 million.