Anti-aircraft gun equipped railcars were used fairly often by Germany in WWII, apparently. No doubt there were similar designs (perhaps some modeled on German prototypes) prepared by the U.S. in anticipation of possible Japanese air attacks on the West Coast.
Not really. During '41/'42, when there was any real possibility of serious Japanese raids against the West Coast, nothing like the necessary numbers of AA mounts was available for such uses. The USN was swallowing production whole in trying to bring the Pacific Fleet up to reasonable capability. When production finally caught up--say, by summer '45--there was not a chance at all of even a token threat. Besides, every car the RRs could get their hands on was being used to move troops, civilians, and supplies.
Fixed installation certainly have AAA capability as did critical areas of some cities, but coastal fortifications lagged behind the times, and most were never even completed. Public fear notwithstanding, there was never any serious chance--or serious intention--of Japanese invasion of continental US soil. The images of Tojo in the White House were propaganda and posturing. The Imperial Japanese Navy had nothing like the amphibious assets necessary to land and support troops on the West Coast even when they were on top of the short-lived domination of the Western Pacific. After the Battle of the Coral Sea, they couldn't even contemplate moving carrier forces toward the US--their abortive attempt at Midway and the Aleutians over-stretched their capability. It's not just a matter of planes and carriers, either: they did not have the underway replenishment capability. Even the Pearl Harbor raid relied on semi-civilian tankers.
Yamamoto's plan was to wear the US down, not take it on. And he, himself, accurately predicted it would fail.
I'm sure that's correct, but John said in his OP that
"I came across a drawing of some proposed military rail cars, one stood out to me, it was a rail car with a twin 40 mm bofors guns that seemed to be mounted on a old crane car base..."
so apparently there was some thought given to it. As the article reference below says, after Pearl Harbor, "As the days and weeks progressed, the initial fear of imminent invasion settled into a long-term commitment to defend the harbor by every means possible. Mobile antiaircraft guns, searchlights and radars were positioned on virtually every hill and knoll overlooking the Golden Gate." This was just for San Franciso, but probably somewhat similar steps were taken in San Diego, Long Beach, Seattle, and other locations. It's not hard to imagine that mobile rail-mounted A/A cars were at least considered. Wartime steam engines operating on the West Coast were modified to be less conspicuous from the air, so at least at the early stages of the war, the possibility of aerial attack appeared to be a consideration.
https://www.nps.gov/nr/travel/.../seacoastdefense.htm
For some facinating reading, check out the history of the Japanese aerial balloon bombing attacks on the West Coast. The balloons were launched in Japan, and then floated via the prevailing winds until they were over the West Coast.
