In the 1930s, new construction techniques allowed for the development of modern, lightweight passenger cars, departing from the heavy riveted steel or wooden-bodied designs found in previous decades.  Materials such as stainless steel and aluminum reduced the overall weight of each car, making them more economical to construct and operate while allowing for ease of maintenance, a smoother ride, and improved durability over time. The smooth-side style was adopted by many railroads across North America for their high-priority streamlined express trains, lasting in common service until the mid-1970s. Today, smooth-side cars are frequently found on excursion trains, while some examples remain in revenue operation as commuter cars or corporate business cars. The streamlined observation car was an evolution of the open-ended observation platform found at the end of long-distance heavyweight consists. Fully enclosed, the car curved to a point, offering a full 180-degree view of the track behind,  with a rear door or window, and space for a drumhead to advertise the service below.