Thank.
I've found info about "train monitors", the armored train of the late 19th century. Another old tech to add maybe.
Also I have found stuff about pre radar, air detection systems:
http://blog.cwam.org/2011/03/before-radar.html Those things rose up and died quickly from ww1 to the interbellum and arent super useful, but it could be interesting to add these for realism sake at least. They'd give bonuses to static and anti air brigades.
EDIT: btw not sure if it has any incidents but the modern airborne tank (sheridan) is labeled as improved like the previous model.
EDIT2: And I think there should be a 1943 or 44 armored train tech. From photos I can clearly see german war trains getting much more armored and armed.
EDIT3: Again, while looking for ww1 anti air weapons I found a picture of German artillery guns fixed on top of trucks. That smells like an earlier SPAA brigade/tech to add.