Susan Fecht @ SciAm: Track Record: Do Major Urban Subway Networks Evolve along Similar Patterns?:
"No two subway systems have the same design. New York City’s haphazard rail system differs markedly from the highly organized Moscow Metro (above), or the tangled spaghetti of Tokyo’s subway network. Each system’s design is the result of many factors, including local geography, the city’s layout and traffic distribution, politics, culture and degree of urban planning."
Nice summary of recent research by Roth, Kang, Batty, and Barthelemy "A long-time limit for world subway networks" in Journal of the Royal Society: Interface (which might be behind a firewall if you don't have library access)
I am interviewed in the SciAm article.