I had another thought:
IMO the most promising option for the role of an armored train is as an armored transport. Building a specialized train to transport a tactical strike force and then support them with artillery fire from protected guns onboard makes using a railroad system which is too high-risk for unarmored trains (because light weapons could either cause a boiler explosion or heavy casualties on any infantry inside who would be unable to fight back until they disembarked) practical.
These are likely to be expensive, if only thanks to their sheer scale. ( I acknowledge that there is also a possibility that largely improvised armored trains, like
this one could become common, but I'm ignoring them for the moment,)
This vehicle also requires a certain amount of tactical competence from the army of the nation that builds it if it is to be used effectively. It is inherently designed for combined arms operations: it's intended as a command and control and fire support (and maybe air defense) node for an infantry / armor assault (or other purpose) force.
It's also a weapon that's tied to a specific kind of infrastructure: long-distance railroads.
I suspect that these are most attractive in theatres which the General Staff in question considers secondary, since one of the main attractions is the ability of the train to support a small force without requiring much additional logistic support. In my last post I noted that the Soviets planned for the possibility of deploying such trains if they ended up at war on their eastern front with China, without being attracted to them for use on their western front with NATO. This time I want to consider an ATL possibility for a theatre making such trains attractive to planners on the other side of the armored curtain.
We need a country with at least some traditional anti-American sentiment to make the possibility of a threat plausible enough that the US might fund a weapon system specifically to defend against it. This country needs to share a land border with the US to make trains rather than the navy the most attractive option to stop a potential invasion/ lead a potential counterattack. And we need a time and place where fear of a breakdown in road-based logistics and a very specific threat level mean that operating relatively small forces independently is viewed as an acceptable element of a robust countermeasure.
The first POD of my proposed ATL is the Eisenhower administration not creating the National Highway System, and preferably investing in rail networks instead.
The second POD is the election in Mexico of a socialist administration that, at least in the US, is viewed as being Soviet-aligned.
The third is the outbreak, somewhere in the American Southwest, of some kind of insurgent/terrorist actions which are viewed, again by the US, as linked to Mexico (again, whether they are actually linked or not is irrelevant.)
This is an admittedly implausible scenario, but I think that if it happened an American Army faced with the possibility of having to fight Mexico with a dubious road network near the border, a fear of insurgent activity in its rear zones, and a fear that any activity from Mexico might be linked to other Soviet sphere attacks on American interests elsewhere, would view an armored transport train as a viable low-cost technological solution.