Metro Deputy General Manager Theodore G. (Tad) Weigle Jr. announced yesterday that he is resigning at the end of the month to become executive director of Chicago's Regional Transportation Authority (RTA).

Weigle, 45, was known to be looking for a director's position in public transportation since he lost out to Carmen E. Turner for the job of Metro general manager in 1983.

He said his new position "gives me a chance to be a general manager -- a number one. Clearly, I'm not that in Washington, and I want to be.

"This is an important assignment. Chicago is the second largest transportation authority in the country after New York , and they made me an offer I just couldn't turn down," he said.

Weigle, who was paid $79,594 a year by Metro, said his three-year contract with RTA provides for a $95,000 annual salary and a "generous" deferred compensation package, which will pay about one-third of his salary on an annual basis after he retires.

He will start his new job Sept. 9.

Chicago RTA Chairman Samuel K. Skinner said, in announcing Weigle's appointment, "We are very fortunate to find a man of the caliber and experience of Tad Weigle. We instructed our recruitment firm to find the best possible administrator and we are very confident that Tad is the right person."

As deputy general manager for Metro, Weigle has been responsible for overseeing much of the day-to-day operation of the bus and subway system, and won praise from Turner for building up "a solid, well-functioning operations management team" and for directing the expansion of the Metrorail system and the modernization of the Metrobus system.

"On the one hand, we will lose the qualities of enthusiasm and creativity that Tad brought and we will miss him," Turner said in a prepared statement at yesterday's news conference. "On the other hand, Tad has an exciting new professional challenge ahead of him. We wish him the very best."

Weigle, who was Metro's assistant general manager for transit operations for four years before being named to the newly created post of deputy general manager in November 1984, said assisting in the reorganization of Metro was his proudest accomplishment.

"When I came here, the operations of the bus and rail were fragmented; there was no real focus. I think I've changed that. We've turned around the attitude about operations around here," he said.

He added that since he has been at Metro, "The railroad has doubled, the ridership has doubled and the bus is infinitely better and more predictable."

Weigle worked in Chicago from 1976 to 1980 as Midwest regional administrator of the Urban Mass Transportation Administration, and was a policy planner in UMTA's Washington office from 1971 to 1975.