The BT 7 tank was the most produced model of all (basically four) models in the series. They all were able to go on railroad tracks as well as roads and terrain. With their thin armour they were fast (in Russian BT= Bystrochodnij Tank= fast tank) and meant to be active in large numbers on the battle field and going by railroad on their own. A small chassis imported from the USA in 1931 was the protoytpe. In BT-7 armour was increased up to to 22 mm, and the hull was largely electro-welded and several variants existed.