No, it's easy to get a single device rated to up to 6kV, here's a transistor rated to 1200V: http://www.suntekstore.com/goods-14005831-igbt_power_transistor_fga25n120_1200v_313w.html
and here's a 6.3kV 7500A monster for train motors:
http://www.infineon.com/dgdl/DS_FZ750R65KE3_2_3.PDF?folderId=db3a304412b407950112b4095b0601e3&fi leId=db3a304325afd6e00126461fd3936974

More typically consumer switchmodes would be 400V mosfets, remember that the peak voltage of 230VRMS is 230*sqrt(2)V = 325V and then you have some overhead to keep the transistor in a safe operating region.  These aren't stacked in the package because then they would become very hard to insulate and need more fancy gate drive circuits:  When you series stack you need an isolated drive for each gate, which in HVDC land usually means a laser through optic fibre onto a mini PV, or a tall transformer.

This is a well known approach (being used for 50 years or more), but I guess this is the first time that the economics have favoured this approach on trains (rather than just a big transformer).  I predict it will become the standard approach within a decade just like switchmodes have replaced transformers for almost all consumer equipment today.