Three members of the pyrochlore series Tl 2 - x Cd x Mn 2 O 7 (0 ≤ x ≤ 0.2), prepared under high pressure, have been characterized by neutron powder diffraction, susceptibility, specific heat, magnetotransport, and Hall measurements. The materials are ferromagnetic, and the Curie temperatures (T C ) slowly decrease upon Cd substitution, as a consequence of the progressive reduction of Mn-O-Mn superexchange interactions. Both electrical resistance and magnetoresistance (MR) are dramatically enhanced with respect to stoichiometric Tl 2 Mn 2 O 7 , due to the significant reduction in the number of carriers (electrons) induced by hole doping, measured from Hall data. In particular, MR(9 T) reaches an unprecedented value of 10 6 % at 120 K for the x=0.2 compound. We have analyzed the conduction mechanism in the region above T C is terms of polaron hopping. The presence of polarons is also evidenced from the deviations to the Curie-Weiss fits above T C . We suggest that the large increase of MR and the observed weakening of the magnetic interactions are both related to the decrease in the number of carriers.