It is not possible to fire a rail shell from the tank main gun. Hitting spacebar does not fire the rail gun shell when it is selected (it fires sniper rifle rounds when the machine gun is selected. It fires impulse rifle rounds when the tank's impulse rifle is selected).
It is only possible to fire sniper rounds and impulse rifle rounds using advanced tank cannons. Main guns (I-III) are unusable with when loaded with rail gun shells.

Factorio: 0.18.42
Krastorio2: 0.9.9

Is clear now?

Remember that u should have the weapon selected or u should be the only one inside the tank to be able to shot

It is clear. It does not work as displayed for me.
I am the only person in the tank.
Explosive rail shell works just fine(selecting it and hitting spacebar does result a shot). I can not try immersite rail shell, as I haven't researched it yet.
Regular rail shell does not fire(once you load it and select it as a ammo)
I have just tested. I can fire an explosive shell, but the regular rail shell simply does nothing. Regardless of the gun you load with it.

The regular railgun shell requires a valid target. You can fire the explosive shell because it doesn't require a target.