Maybe I just haven't been looking carefully enough and you've probably explained this already somewhere but... where did the Alta California railroad idea originate in? It would also be fun to read about your inspirations for it.
When I was a kid, my local library had a copy of Bruce McGregor's excellent book "South Pacific Coast Railway" in amongst the boring 'general interest' train books (think titles like "WORLD OF TRAINS" or "American Railroads", books so vast that they say nothing and show little). This was the first ever serious train book my kid eyes had seen, so I checked it out and brought it home.
Inside it had route maps, endless beautiful old glass plate negative photos and surprisingly accessible, engrossing writing to suck you into the history of the Bay Area California narrow gauge line that connected a Ferry Terminal on Alameda Island with Oakland, San Jose, and Santa Cruz over a challenging mountain pass.
It really ignited my imagination, especially because the little 3' narrow gauge line was standard gauged following the catastrophic damage it received during the 1906 earthquake. You can still ride portions of the SPC on Amtrak's Capitol Corridor from Newark -> San Jose. You can also ride a portion of the Santa Cruz branch on the "Roaring Camp" railroad behind an ex-Santa Fe CF7 diesel through the redwoods that takes you right to the beachfront amusement park. I got to ride the latter at a young age and gave me an affinity for trains going through redwood forests.
Paging through this book made local history and surviving views, buildings, and infrastructure come alive. I often biked down Encinal Avenue in Alameda as a teen and imagined being paced by those little teakettle 4-4-0's hauling wooden coaches filled with overdressed commuters in lavish Victorian attire.
I also always had an affinity to not only model real existing railroads, but also modeling fictional railroads my friend Ian and I made up. We developed a believable freelance railroad as tweens, the "Falcon Pacific" which connected with the SP interchange at Redding, CA and went over challenging mountain grades to the town of Crescent City, CA and south to connect to the NWP in Eureka. In retrospect I'm pretty surprised we went that deep and logical as kids. We painted up some old crappy F units in a handsome dark blue paint job and used Woodland Scenics dry transfer decals to make wobbly lettering down the sides of the units. It was a blast. I still have two of those engines from back then.
In my mid-teens I had the first dedicated layout space in my life. Our 1927 split level art deco rental home in Alameda had a garage precisely large enough to fit a Ford Model A, and no modern car. So the parents let me build it out into a layout room and workshop. It had original checkerboard maroon/cream linoleum tile floors, finished walls and ceilings which kept the dust in check.
There I developed the "Mission Valley & Pacific" Another freelance railroad that used the mostly abandoned original California Southern right of way out of Riverside and down to San Diego. It had a very handsome dark blue paint scheme with red/orange wings similar to the Louisville & Nashville, but much smaller in size, similar to the SP "black widow" wings. I painted up a couple of locomotives and a few freight cars (I still have the freight cars) and the MV&P was the home road on my large modular 1 car garage layout for a few years.
When I moved out of my parents place into my new Live/Work space at 19, I continued the MV&P concept, but stalled out. It didn't have the kind of traffic I was interested in modeling and moving the modular layout I used to have damaged it too much to reuse, so it gathered dust and interest waned. Years passed and I didn't do much other than commission work for others to pay bills.
In 2015 I was sitting in a cafe in San Francisco with Alexander and I'd recently begun to discover Mexican railroads. We brainstormed a bunch of new ideas, including an evocative unused concept where Mexico still owned the pre-1849 Alta California territory and imagined what that'd be like culturally, politically, transportation wise. Out of that we knew the name for the future railroad had to be the Alta California Railway, not least because "ALTA" is a fantastic reporting mark to put on rolling stock, and is still unused by the AAR, so it was available to use.
(An important consideration when planning a fictional railroad is to NOT have identical reporting marks to existing railroads. For instance if your railroad was the Sierra Pacific, you cannot use "SP" because that's the Southern Pacific. You'd have to look at the list of AAR reporting marks and develop one that's not used, like SRPC. An exhaustive list of all North American railroad reporting marks is here: https://en.wikipedia.org/wiki/List_of_reporting_marks:_A use CTRL+F to search for ones you're considering, and if they're not there, you're good to go.)
I worked with Alexander and Tom Coletti to develop the ALTA look and feel. We fleshed out its Oakland to Guaymas (and later Guadalajara) mainline, we worked together to figure out how it fits into the arc of REAL history to make sure it doesn't step on too many toes to still remain plausible in our reality. We also had to align our railroad against competitors and with allies to survive "the Octopus" SP railroad that strangled California politics and commerce from 1860-1930.
So it all returned to that book I checked out of the Library as a kid, the South Pacific Coast. In a fashion akin to how Ursula K LeGuin writes her alternate realities: what if we changed just ONE single knife-edged decision in history and then roleplayed how it would have unfolded in our own reality? The one thing we changed was whether or not Silver magnate and railroad baron James Fair sold the South Pacific Coast to the Southern Pacific in 1888. Then we extrapolated all the rest of history forward, which you'll be able to read this summer/fall in an installment history series here.
Along with the historical considerations, developing a corporate image for the Alta California remains a constant source of fun. Between Alexander, Jay, Tom, Myself, and others we've developed everything from complete rolling stock and locomotive rosters to travel posters to dining car menus (we even tested out the recipes and created our own from scratch!) and it's blossomed into SO MUCH more than crayoning a route map and painting up some diesels. It's now becoming a cohesive work of art with its own lore, and an ever expanding collection of rolling stock in addition to the layouts current, past, and future.
It's been multiple different hobbies in one with the model trains being the anchor. Doing polished, period correct design work for both 1:1 physical paper items and 1:87 scale model items to build a believable reality in which James Fair's little narrow gauge became a modest, Western Pacific style standard gauge empire connecting the Bay Area of Alta California with Guadalajara Mexico from 1888 forward has been an incredibly rewarding challenge.