I think the turbine would have to be close enough to slow the airflow boundary layer increasing drag on the moving train and/or cause tbe train to need to re-accelerate slowed air, and that would have to be uncomfortably close turbines. We’re not even getting into the weeds about the drag cause by the tunnel walls and any structure protruding from it, or how aerodynamic the train is in the first place yet.

Cars are as you point out a completely different animal, and the effect of any turbines would affect the next passing car only if a) the car were close enough to benefit from the “draft” of the preceding car, and b) if a is true, then it would lose energy re-accelerating air energy lost to the turbine.

I actually did give it more than a passing thought, and even if it’s just an esoteric discussion, the energy losses wouldn’t be worth calculating, and the gains minimal.