How hard would it be to add an option to change the sprite for the armored locomotive to look like the old Diesel Locomotive, you know, the one that kinda looks like a rolling bunker?

The armoured locomotives are just using a deepcopy from vanilla locomotive, with property changes after. There are no graphics/animation changes in my code. So I assume any code changing sprite of vanilla locomotive can easily be applied to the armoured locomotives as well.

There is an existing mod Old Train. I have not tested with that. Guess it is replacing the sprite of vanilla locomotive.
If it does not automatically apply to armoured locomotives, I guess it can be done with very minimal additional lines.

Would be cool. I do like the idea of the armored train looking different from the basic one. Up to you of course.

Okay, it sounds reasonable.
Though I prefer to have this mod in small size, so putting these extra graphics to be in an external mod.
I don't have much time these weeks. So feel free to do it yourself (or some fellow modders). Guess it should not be too difficult.

Sounds good. I'm not playing actively right now, but I do plan to return.

Your request is just fulfilled! ;-)
It is released as a DLC, named under Schall Armoured Train: Nostalgia.
You can freely select to apply the 0.12 graphics to armoured locomotives, vanilla locomotive, nuclear-powered locomotive, or any mix of above.

PS: I have looked into the existing mod "Old Train". It has all the sprite sheets inside, but not doing what I expected. So I make the DLC instead. Have fun!

Ty ty!