With a Tl coverage of one monolayer, a 2 x 1 reconstruction is formed on the Si (001) surface at room temperature. In this study, low-temperature angle-resolved photoelectron spectroscopy (ARPES) data reveal four surface state bands associated with this Tl induced reconstruction. Calculated surface state dispersions, obtained using the "pedestal + valley-bridge" model, are found to be similar to those obtained using ARPES. Inclusion of spin-orbit coupling in the calculations is found to be important to arrive at these results. A known effect of the strong spin-orbit coupling is the reluctance of the Tl 6s(2) electrons to participate in the bonding, i.e., the inert pair effect. In the calculations, inclusion of spin-orbit coupling results in a similar to 5 eV downshift of the Tl 6s(2) electrons.