Great question!
For armored trains that are not simply artillery guns on rails, I think it would be cool to have them as units on the map. This would make them vulnerable to attack, but considering the wide range of armament on these types of trains (12.7 mm through 76 mm at least), they are also appropriate to the scale of ToI.
One could attempt to create a whole new type of unit--Armored Trains?--that could have varied Armor and Firepower ratings in accordance with the consolidated reference stats, or one could make these scenario-specific units. I'm agnostic about this approach, but my key recommendation would be to ensure their firepower is calibrated to the other unit statistics. For example, an armored train with 12.7 mm MGs is going to be a much different unit than one with 76 mm tank turrets (and there's variations between these extremes).
The rail artillery guns, in my view, are best treated by the artillery card mechanic of ToI, but I can envision all kinds of fun scenarios in which armored trains with guns in the 12.7 mm through 76 mm range could play a cool role as on-map offensive or defensive units. This gets to the interesting nuance of Katyusha and Nebelwelfer guns, which to me are right on the border of being off-map artillery or on-map units that may be destroyed.