I unloaded rail cars with nitrogen for some years. It's the same type of fluid transport except mine was more complicated because I had to ensure the phenol was melted and stayed fluid.
What you do is that you calculate the nitrogen pressure required to displace and move the Phosphoric Acid from the rail car to the truck. The major problem with this system is avoiding a nitrogen "breakthrough". Unless you have a means of capturing and containing the nitrogen that follows the phosphoric acid into the truck. There are various ways to mitigate this event and you should develop a detailed P&ID showing how the operation can be carried out in a fail-safe manner. You have to calculate the entire line pack as well as the volume of the rail car. This will give you the amount of nitrogen needed to displace the phosphoric acid. The ultimate pressure of the nitrogen in the rail car will be the sum of the hydrostatic head + the line & entry/exit losses the phosphoric acid has to overcome.
I don't particularly like this fluid transfer method because it is subject to some safety measures. But some consumers are cheap and won't invest in a more secure pumping operation.