A FEW kilometres outside Frankfurt, nestled in the rolling hills of the German countryside, is the charming little town of Russelsheim. In the evening, colourful barges drift slowly past on the peaceful River Main and townsfolk stroll along the narrowstreets, passing friendly greetings to friends emerging from the many quaint restaurants and beer halls. Only in the broad light of day and with careful scrutiny do you begin to notice that half hidden behind the centuries-old buildings is a sprawling 1.5 km-long industrial complex that houses Europe's largest single automobile plant. Russelsheim is home to Adam Opel AG, one of Germany's leading manufacturers of motor vehicles. Last year alone this single plant turned out more than 1.6 million units, making up the lion's share of the company's annual world-wide production volume of more than two million. The factory and the town peacefully coexist. Indeed, more than 8,000 residents, 50 per cent of whom have training from univer sities or technical colleges, find employment with Opel and, in turn, the town's commercial and trade sectors depend heavily on the factory for their business. The emphasis on cleanliness and environmental concerns that typify German factories means that any negative effects on the town are minimised. ''Our company and this town have been partners for years,'' said Alfred Korbel, manager of communications for Opel's technical development centre. ''There is a great deal of mutual respect and interaction, which makes it work very well.'' Mr Korbel and his associates are proud of the emphasis Opel places on research and testing. ''In our technical development centre, virtually every component of our vehicles is exhaustively tested for safety and endurance,'' he said. ''This makes Opel vehicles among the safest and most reliable of any in the world. ''That is also very important to us, because, after all, almost everyone in Russelsheim drives an Opel.''