T-28 Model 1938, or T-28B, was improved version of T-28, with new gun, gun stabilization system and engine. According to one source, during Winter War over 200 T-28s were knocked out, but only 20 were totally destroyed. The rest were quickly repaired and send back to the fight. Some of T-28s were repaired durign the course of war even five times.
T-28 Model 1938 was improved after losses sustained during Winter War, with installation of better 76mm gun and new engine. T-26E or T-26C was variant with increased armor thickness (on front from 50mm to 80mm). "TU" stands for "tank upravleniya", "command tank" as it is equipped with a hand-rail frame antenna.
NOTE: Addition of TU is my personal choice. Officially it was just T-28 with hand-rail frame antenna.