Found them, thanks.
After watching the replay and comparing to countless attempts by me, I would say to achieve a BV in this scenario is more dependent on RNG (mostly combat rolls, but also weather - at one reload, two of the three tank units were actually out of supply and stuck in the mud with very little mobility before the first turn) than on actual tactics. In this replay, not only did the tanks never lose a single step in all those overruns, but the veteran unit has actually been ejected from the first rail city, something that never happened to me in maybe 20 attacks. Also, in about every 2nd or 3rd attempt, the 3 tanks can't remove the first unit at the bridge, meaning you can just as well restart the scenario at that point. In quite a few scenarios, tanks also lost steps (not just suppressed).
Compared to Stalingrad, where I could achieve a BV in a mere 7 turns without buying any of the OKH reserve units (buying some specialist steps and some armor steps though), and get a one turn early BV with even a number of bad combat rolls, this scenario has IMO a too tight victory condition. Using a good strategy, a Terek victory in 7 turns is possible even with a string of unlucky combat rolls (like tanks repeatedly getting stuck on their first attack), but 6 turns seems to be achievable only with a lot of luck on rolls.
I've also been trying an alternative strategy, where after removing the bridge unit one of the tanks drives 3 or 4 hexes southward, blocking the veteran infantry from settling into the first rail city. The whole scenario develops a different dynamics, where your advance is mostly checked by having a bunch of infantry in the back, trying to cut your rail line and even trying to capture your supply base. But as far as 6 vs 7 turns victory goes, it didn't make a difference.