How are T cells transported?
The progenitor cells of T-cells (they do not develop into T-cells and show their characteristics before reaching the thymus) are mobilized into the bloodstream and enter the thymus from it by crossing the endothelial barrier. The figure shows schematically the development of the precursor cells from hematopoietic stem cells (HSC) and the migration of the precursor cells (it is from the paper linked below):



See this review for the details (if you have problems accessing the article, let me know):


The long road to the thymus: the generation, mobilization, and
circulation of T-cell progenitors in mouse and man.