Rod says he's spent the past 23 years creating an imaginary U.S. city in the attic of his Los Angeles mansion, complete with intricate details and over 100 feet of railroad tracks. Who knew that's what he was up to when he wasn't making music or performing live?!?

The British rocker finally revealed his secret passion in an interview with Railway Modeller magazine, telling the outlet he even took his hobby on tour, working on skyscrapers and other scenery in extra hotel rooms specifically reserved for his construction project.

Rod says he prefers creating structures and scenery as opposed to laying down railroad ... and his whimsical town features warehouses, high-rises, mills and different landscapes.

The railway infrastructure is pretty detailed too ... there's rail stations, tunnels, bridges and, of course, tracks. There's even cars and ships.

Rod's city is so intricate, he says he made sure every pavement stone in town is a slightly different color. He used black chalk to create cracks and even put fake garbage in the gutters and incorporated rust.

As for the roots of his hobby ... Rod recalls seeing a model railway in a window display when he was growing up in England, and he wondered how he could get paid to build one.