Schematic showing the exact routes taken for the trafficking of women, children and men. The indicated arrows show the routes, and were self-made based on following maps:
http://www.fas.org/irp/threat/754727.gif
http://traffickcharting.wordpress.com/page/4/ (PBS maps)
http://www.moraviaschool.org/tfiles/folder2410/Map%20of%20Human%20Trafficking.jpgAs is clear from the map, The main transit countries are the USA, Spain, Italy, Albania, Bulgaria, Russia, Saoudi-Arabia, India, Thailand. Note that the routes run along large cities (due to the presence of airports, housing). In the http://www.moraviaschool.org/tfiles/folder2410/Map%20of%20Human%20Trafficking.jpg Poland too was noted as a transit country, yet it seems that this country will probably just be driven trough by car. Most routes are airplane-routes, yet some (mostly the short ones, ie in Europe, ...) are probably done solely using a car, hence not requiring the presence of a large city. The major cities indicated that are not capitals are 

USA: Los Angeles and Houston
Ukraine: Odesa
China: Chongqing
Australia: Alice Springs
Note that it is not entirely certain that these cities are indeed major hubs, yet from the source maps, it seems logically that they are.
Own work
Genetics4good