PRINCETON, N.J. (AP) _ Shareholders of Pullman Co. and Peabody International Corp. on Thursday approved a merger that is effective at midnight.

The Pullman-Peabody Co. will begin trading Friday on the New York Stock Exchange under the ticker symbol PMN and will be abbreviated in Associated Press newspaper stock tables as PulPe, with a footnote (n) designating ''new.''

Thomas M. Begel, who will be chairman and chief executive officer of Pullman-Peabody, said, ''We are excited about the Peabody businesses which will form an integral part of the merged companies' strategy of continued growth in the manufacture of specialized engineered products and systems for the industry.''

Pullman, with sales for the fiscal year 1985 of more than $300 million, owns companies including PTC Aerospace, a manufacturer of seats, galleys and food service systems for commercial aircrafts and Trailmobile, which produces tractor trailers.

Peabody, headquartered in Stamford, Conn., manufactures fluid material handling equipment, automotive stampings and food service equipment. The company also designs and constructs environmental cleanup systems.

Peabody had sales for fiscal year 1985 of more than $350 million.