Piccadilly Circus, one of the busiest Tube stations in central London, was completely reconstructed in 1925-28. Built to replace the inadequate surface-level booking hall and lifts, the new circular hall could accommodate 50 million passengers a year. Holden transformed this bleak cavern into a welcoming underground environment by creating what he called an 'ambulatory'. His design was more like a high class shopping arcade than a station, with window showcases and marble wall panelling. Piccadilly became the jewel in the Underground's crown, much admired by visitors to London.
Holden was already working on an even bigger scheme for a new Underground headquarters. 55 Broadway was the largest and tallest office block in London, built on an awkward triangular site over St James's Park station. Holden decided on a cruciform plan which gave street level access to the offices and station from both sides. The mass of the building was stepped up to a central tower, giving maximum daylight to all floors. This was London's first taste of American-style office architecture, soon known as 'The Cathedral of Modernity'.