Well I was running a Workshop Scenario and noticed the tank car it called for on a drop, designated by a car number, had the dreaded 543210. Which means it was on all the tank cars. Well tried researching this a while back for another car but didn't have much luck. Very few posts covering this problem. So I figured now was the time to figure it out myself.
To make a long story short; in the tank_car..geopcdx the "primarynumber" textures did not have primary number textures assigned correctly. The entry just called for a "number_" texture. There was no textures named "primarynumber" in the tank_car texture folder either. So to make it as simple as possible without adding another separate set of numbers in addition to the current set of numbers I just copied and pasted the number textures from the BoxCar50Ft in the Freight folder to the tank_car texture folder as the boxcar folder already had all the number textures named as "primarynumber_".
You don't have to redo the entries in the copied and pasted number textures. Though they have a entry of "RailVehicles\Freight\BoxCar50Ft\Default\Wagon\Textures\primarynumber_" that doesn't affect anything unless you are going to reskin the number themselves.
Corrected the entries in the tank_car.geopcdx as shown in the illustration.
Then I corrected the tankcar_Digits.bin entries to match the name of the new number as shown in the illustrations.
Note: At the top of the tankcar_Digits bin file page there is this entry: "Name :type="cDeltaString">Cement Digits</Name". However there is no need to correct this as long as the file name itself is "tankcar_Digits" because that is what the tank_car.bin calls for.
As you can see, it was successful.