....What Luigim said.......
and
https://tanks-encyclopedia.com/coldwar/USSR/MT-LB.php
MT-LBM (izdeliye 6M) Russian IFV, 30 mm autocannon
AND.....
That is a tracked carrier with a turret and a long autocannon and 6 road wheels.
An MT-LB has 6 road wheels that look just like those road wheels
as does those things whizzing by on the flatcars.
If you know of another Russian tracked carrier with 6 roadwheels and a big assed turret with a long autocannon near the middle back end I'm interested to hear what it is. A bmp has 6 but not quite like those but MAYBE what were seeing is a new BMP variant. The BTR-82A has a turret sort of similar but it's more forward on the hull and has tires
MAYBE it's all maskirovka , They only have a few protos but move them about on trains to impress people...
but what went by on those flatcars looks
a lot like....
