A small supplement - early carriages on the steppes may have been cattle-drawn, and may have facilitated the wanderings of the steppe people. As you have mentioned the Sintashta, you are already aware that they got there after long years, centuries, spent in wandering, first, on an eastward wave out of their earlier westward movement into Europe, and after the Beaker People are found, then, by various paths, not all uniform, to the Sintashta location. It is possible that the Tokharians, an apparently Centum people, went east, instead of west, like all other Centum people.