4548 Transformer and Speed Regulator Certified SetThe LEGO 4548 1 Transformer and Speed Regulator is a 9V Train set accessory that provides power and speed control for 9V trains. It was released in 1991 and includes a transformer and a speed regulator with 12 speed settings (six forward and six reverse). Unlike the previous 7864 Transformer Speed Controller 12V, the 4548 uses an external 9V power supply, making it a unique 9V set that doesn't rely on batteries.