Norfolk Southern Corp., a railroad holding company, yesterday reported net income of $99.8 million ($1.59 a share) for its first quarter.

The performance was 3.3 percent lower than the company's $103.2 million ($1.64) showing in the first quarter of the previous year. First-quarter railway operating revenue of $862.2 million was 2.5 percent lower than the $884.7 million achieved in the first quarter of 1984. "We are very pleased with these first-quarter revenues and earnings," said Robert B. Claytor, chairman and chief executive officer. "The severe weather throughout much of our territory contributed to the decline in revenues, but we maintained tight control of expenses."

* James Madison Ltd., a District of Columbia bank holding company, reported first quarter profits of $823,157 ($1.00), up 26 percent from $652,697 (79 cents) in the same period last year. Total assets increased 12 percent to $308.7 million, compared with $274.9 million on March 31, 1984.

"These first-quarter gains reflect the successful growth of James Madison Ltd. and its principal subsidiary, Madison National Bank," said K. Donald Menefee, chairman of the board and president.

* ERC International of Vienna, said first-quarter net earnings declined 47 percent to $219,000 (7 cents), compared with $411,000 (13 cents) in the same quarter last year.

Quarterly revenue increased to $10 million compared with $7.8 million the year before.

The company said the profit decline reflects continued costs of product development and the establishment of a national sales and service organization for its InterCAD subsidiary.

ERC provides professional technical services and products in areas including engineering, information sciences, logistics, energy and computer-aided design drafting.

* Computer Entry Systems reported a 50 percent increase in first-quarter net income to $290,000 (7 cents) from $193,000 (5 cents) in the same period of 1984. The Silver Spring company said its total revenue reached $8.2 million, compared with $5.5 million in the first quarter of last year.

Computer Entry makes cash-management systems for banks, utilities, retail, insurance and publishing companies.