Having worked unloading coal tar pitch tank cars I recall having to open the vent valve in the dome or else the car could literally collapse as the pitch was drained out of the bottom fitting, if it would flow at all.
Draining the car increases the "air" pocket volumn above the fluid creating a vacuum that increases as the liquid level drops putting stress on the car.
As the volume (V) increases as the tank is emptied the pressure (P) must decrease proportionally creating a vacuum to balance the equation. This can and has collapsed many storage tanks. Not sure if a rail tank car is strong enough to avoid collapsing as a full tank is drained to empty?
In the case of a pressured gas car, the risk of the positive pressure dropping into the negative range is low as the tank was pressurized prior to unloading.
Yes, I am a nerd.