A 20 percent boost in profits pushed the Southern Railway Co's 1977 earnings past the $100 million mark for the first time, as the Washington rail-roads's revenues soared past the billion dollar mark.

The Southern reported yesterday it earned $107 million or $7.04 a share on revenues of $1.14 billion. Earnings were up 20 percent, revenues nearly 11 percent.

L. Stanley Crane, Southern's president and chief executive, predicted the favorable economic factors that produced the record results will continue this year, despite the coal strike.

Southern's coal hauling revenues last year increased 30.2 percent, growing faster than any other business. Crane said the coal that isn't being moved now will have to be moved later and the strike is not expected to significantly affect coal carloadings for the full year.

The railroad said its revenues were up for all commodities except one - aggregates and non-metallic minerals - and total carloadings were increased 5 percent.

Operating costs for Southern increased at about the same 11 percent rate as revenues, growing from $745 million in 1976 to $829 million.

Crane said Southern's fourth quarter not declined by 4.6 million to just under $20 million, mainly due to accelerated maintenance work and the coal strike. Revenues for the quarter were $288 million, and earnings a share amounted to $1.61 the prior year.

Maintenance costs for the quarter of $107 million were the highest of the year, with right of way maintenance costs up 22 percent to $56 million; revenues climbed only 7 percent because of the strike, the Southern said.

MCI Communications Corp. of Washington reported net income of $2.6 million or 12 cents a share on revenues of $18.9 million for the three months ended Dec. 31.

During the same period a year earlier, the company earned $1.3 million or 5 cents a share on revenues of $16.8 million.

MCI said its latest quarterly earnings included $1.5 million from the sale of proprietary software, while a $1 million extraordinary item was included in the previous year's results.

For the first nine months of its business year, MCI reported earnings of $4.5 million, compared with a loss of $170,000 on revenue of $45 million last year.

C.A.C.I. Inc. of Arlington reported a 92 percent increase in earnings on an 18 percent increase in sales for the six months ended Dec. 31.

The research and services company had revenues of $5.9 million, up from $5 million and profits of $289,000 up from $150. Fully diluted earnings per share were 28 cents versus 15 cents.