Berlin is characterized by its Plattenbau architecture, a legacy of its communist past, with the Eastern part of the city having the most examples. The SED's repressive regime operated a massive intelligence and security force, the Ministerium für Staatssicherheit, to suppress opposition. Berlin's population is diverse, with many artists and musicians contributing to its alternative culture. The city is also known for its vibrant graffiti and urban art scene, extensive public transport system with distinctive yellow U-Bahn and red & yellow S-Bahn trains, and a variety of travel destinations. The RAW-Gelande, a former railroad factory, has transformed into a creative space with cafes and restaurants.