The tube has shorter times between trains than the New York subway. For example, Picadilly/Central/Victoria line trains run at <120 second headways on the central section. You couldn't control the gaps between trains precisely enough to demerge and remerge trains once you account for random factor of passenger loading and unloading and changing gaps between trains as they accelerate and decelerate between stations. Better to build a whole separate line, in fact this has been done already, the Victoria line is pretty much the express version of the Piccadilly line and likewise Crossrail will be the express version of the Central line.