ThinkPad W series
The W series was developed as a mobile workstation line, which was supposed to directly replace the T series 'p' models.
As the W series was simply just another tacked-on name for the T series 'p' models, the only difference that separated it from the T series remains the same (a faster graphics controller). Some models of the W series' were also known to have 4 RAM slots, which was not the case for the earlier T series 'p' models.
The W series was discontinued after some time, and was then succeeded by the P series.