Fabulous stuff, Xen! I particularly like that your story arc begins as HAE hits the inevitable postwar order duldrums. Overall, this is a true work of art 
Thanks! It was, as always, a lot of fun figuring it out, especially the parts involving learning about all the RW bits and pieces behind it all. The story of BL could be made into a TV series to rival Game of Thrones... I doubt GoT has nearly as many twists and turns and oddities...
Now a few nerdy detail questions (in no particular order):
1 - In aviation, Hoffar/BC Leyland emerge as undercarriage design/construction specialists - so, effectively becoming the Héroux-Devtek of BC?
Essentially, yes!
2 - Did the Lumby plant have extensive casting facilities? (This is prompted by that interesting mix of aero engines and locomotives c.1950. Or were Lumby's cast components brought in from elsewhere?)
This is a good question. I'm sure there's some casting capacity, as I expect (almost) everything to make a Heron would've been in easy reach. As to just how much? I'm not sure yet. I'll have to do some mapping work with topos of the area to figure out just how much space there is to work with for the complex in the area adjacent to Lumby proper for the "big part" of the complex, and what goes where. Stamping and assembly are certainly in the main complex (I imagine in a sort of linear arrangement). The engine plant is about 4 miles to the west of town, in Dure Meadows, and in Lavington is the Okanagan Glass Works, which does all manner of glass products including for BCRL, and the Lavington Planer Mills, which does furniture (but I suspect also woodwork for the high-end cars, Wolseleys lately mostly, maybe the top-tier models in the Rover lineup)... in my notes for BC Rail's Lumby Line I have a non-specific "industrial sidings" mentioned at Coldstream Ranch, that could be related or not. Of course, there could be facilities located further away, as long as its along the railway...
As far as casting goes in specific, since you brought the locomotives up: I'm not sure what kind of frame the 65-DE-19A had, I just really like its looks, but I know EMD built their S and N series switchers in the mid 1930s with both cast (SC) and welded (SW) frames... the welded having won out, as the second series only came as SW1 and NW1. If welded, I see no problem, there's plenty of space... if cast frames, well, Vancouver Locomotive Works in Surrey* could have supplied those, as VLW wasn't really doing diesels yet** so wouldn't have seen Hoffar's 65-ton shunters as any sort of competition to their own mainline machines.
* adjacent to BCR Liverpool Yard [RW: CN Thornton Yard], roughly a million square feet in the riverfront from Bolivar Creek west to about 128 St, and however far inland that goes; manufactured steam locomotives back in the day, then later electric locomotives and EMUs (that's where Kálmán Kandó was employed in developing BC Rail's electrification in the 1920s/30s, but that's another story for another day).
** Not only did VLW supply the bulk of steam locomotives used by the BCR, from 1929 to 1969 VLW built *all* the electric locomotives and EMUs used in BC, but after 1972 a grand total of only 76 electric locomotives were built by VLW, having switched over to diesel and electric multiple units... most locomotives since then all come from abroad. Diesel production started in the later 1950s with the DE-6-33 family, a North Americanised version of British Rail's "Deltic" (so not dual-cab, but single-cab A units and cabless B units like most mainline diesels made in North America in the 1940s and 1950s). VLW did rebuilds of GE U-Boats for the NWP*** in 2000, and I *think* they did build a small number those under licence in the 1960s/70s (but I don't recall offhand), but after the end of steam locomotives, aside from the Deltics their focus was electrics and MU sets.
*** NWP = North Western Pacific Railroad, formed in 1998 through the merger of the Southern Pacific Railroad and the Chicago, Milwaukee & North Western Railroad (itself the result of the previous merger of the Milwaukee Road and the Chicago & North Western). NWP has a minimal presence in BC: the line from Mission south to Huntingdon/Sumas, WA (RW: CPR) is their only presence in Western BC. In the east, the Milwaukee Road acquired the line from Coeur d'Alene, Idaho through Jennings, Montana to Elko and on to Michel after buying the Crowsnest Southern Railway in 1911 (the Elko-Michel section was closed in 1929), and this line was extended from Elko to Flathead when a coking-coal mine was opened there in 1961; this line also has a branch line to another mine at Outlier Ridge. The Milwaukee also acquired a line from Coeur d'Alene to the BC/US border at Metaline Falls, WA/Nelway, BC in 1916 through the acquisition of the Idaho & Washington Northern Railroad, and then in 1922 absorbed the Kootenay & Pend d'Oreille Valley Railway which went from Nelway to Castlegar. This is the extent of the NWP in BC, though it is of course a vast operation in the US (with HSTs in California, plenty of electrification, and one of the very few major railways in the US to not delegate passenger services to Amtrak).
3 - For automotive, Canada stays LHD but the Dominion of BC never goes through its OTL 1922 switch from RHD? (That certainly simplifies auto exports/imports to and from the Antipodes, Japan, the UK, and whole bunch of former British colonies. Nice! Does your ATL ROC also stay on the left after 1946?)
Yeah, BC stays RHD, so I imagine the border crossings are a bit interesting to accomodate the crossovers (I'm picturing something like traffic lights/intersection at smaller/less busy crossings, and over/underpasses at the busy places.
ROC: never occurred to me that they were on the left, though makes sense given they were under Japanese rule. I don't know . I might rethink Asia a bit: for Africa, the Ghana plant covers the LHD part of the continent, and RZ covers the RHD countries; could be that Chonburi Motors is covering RHD Asia and China Rover-Leyland are supplying the LHD world, in which case... yeah, I think ROC switched over.
4 - When British Rover Group's truck and bus divisions were sold off in 1986-87, did BCRL lose its rights to produce those lines?
I haven't yet done much (i.e. any) work on Leyland products made in BC, but I expect that since DAF has never had much if any presence in North America, I think they would have been amenable to letting BCRL keep building anything that was already in production in BC at that time, and then independently further developed as needed, for the North American market (in which I include the West Indies)...
If so, what replaced them in our domestic market?
Well, trucks/HGVs are well covered, with the GM-owned Bedford plant in Blaenau, and Hayes and Pacific in Vancouver.
A - Still on busses ... for city transit busses, did the Dominion of BC use double-deckers? Some of the Leyland busses had an odd 'notch' in their rears to clear engines. I wondered if such engine bays might eventually evolve into flat diesels? (I'm a fan of the bus and Swedish military use of Volvo's flat-6 diesels.)
Busses - although an interest of mine, too - aren't something I've done much work with yet (aside from city bus routes in the GVRD and Greater Blaenau). So, off the cuff development: Leyland and Bedford were the big postwar suppliers (for Bedford, I'm envisioning RHD versions of GMC busses, I love the Fishbowl too much not to do that), with maybe some local transit companies (prior to the formation of the Urban Transit Authority) having bought from other suppliers (would Prevost, Flexibl, Fageol, Flyer have bothered to do RHD just for BC? The Canadian ones, maybe, after the British Columbia and Canada Act of 1957 which created the free trade zone... BC after all is much more populated *there* than *here*). Now, I'm thinking that after Volvo acquired Leyland Bus, maybe they hoped to get their own piece of the North American pie, so BCRL had to discontinue production of Leyland busses... now, I know that the Japan Auto Terminal was opened in 1986 (on the south shore of Lulu Island, near Finn Slough/Fraser Wharves), after the trade agreement between BC and Japan on motor vehicles, so I'm thinking that Volvo forcing the end of Leyland busses in BC opened things up for Japanese manufacturers (and they have some cool busses there!).
Continuing: since it's GM, and since there's no restrictions on trade between BC and Canada (effectively a single market arrangement, more or less... things are still in the handwavey
quod assumpsit, assumpsit state), at some point GM decide to consolidate their bus-building operations, so production of busses in Blaenau ended it 1979, and BC was supplied from the St-Eustache plant. Et cetera et cetera as things happened *here*, St-Eustache is now Nova Bus, and still producing RHD busses for BC (and though in a roundabout way, Volvo did end up getting into the BC bus market in a major way).
So there we have it: transit busses nowadays all are sourced from abroad, primarily Canada or Japan...
B - In Alt BC, is there any need for a relative modern 2.5L L4 engine? (I note that OTL petrol Rover 2.5L doesn't appear until 1985 - a year after Rover's diesel 2.5L.) If yes, read on ...
I was working on a notion for an ALT CA auto industry involving AMC. My idea was developing an mid 1970s 4-cylinder (post '73 oil crisis) on the basis of half of an AMC V8. The starting point was the 1970 5.0L (304 cid) V8. (My assumption was that 'cutting down' the AMC 6-banger to four resulted in too large a displacement - a 258 derivative producing a 2.8L/172 cid 4-cylinder.)
Adapted for your Alt BC RHD, my 304 derivative would 'recycle' the complete LH head from the V8 - which puts the exhaust ports on the passenger's side with the carburettor (later FI) placed on the driver's side. The pistons, rods, etc. would be identical to the 304 but, obviously, the block and crankshaft would be new designs. The result would be a relatively economical 2.5 L 'slant-4' producing about 105 bhp (or 75 hp SAE Net for the US market).
I doubt that I'll proceed with my ALT CA auto industry concept. So, if this of any use to Alt BC ...
First off, quickly: yeah, this is great - I've been trying to brain up more motivations for the BC Leyland/AMC cooperation of 1979 (which was partly motivated by my love of the look of the late 2-door Matador, plus a general soft spot for AMC)... sure there was the engine swap that had some AMC models having Kenosha-made Rover V8 as an option (outside of California), and production in BC of the Matador as the Wolseley Matador (2 door) and Wolseley Barcelona (4 door), but although the AMC inline 6 went BC-wards, there wasn't much use of it or the AMC V8 prior to BCRL's acquisition of AMC other than in the 1979-1981 Rover 4200 and 5900 (Rover P8 design, LHD only with AMC engines for US only sales) - it was used more after 1984, of course, particularly in the Eagle Rambler (as the AMC Eagle was renamed) and the Eagle Concord, but not past 1988...
Rover's 2.5L petrol and diesel engines were, AFAIK, used only in Land-Rovers and Freight Rovers; the first 2.5L in Rover cars (*there* was the KV6 V6s from 1997), and I didn't think I was missing it, but your AMC-derived slant-4 idea might just fit something...
The Austin Allegro came to BC in 1973 (without the square steering wheel!) with either a 1.3 L A-series I4 (69 bhp) or 1.75 L E-series I6 (76 bhp) engine, replaced by the Mk II in 1975 with the same engine options. The Morris Marina came to BC in 1973 as well (but named Austin Okanagan, which name goes back to 1966 on the wagon versions of the 1100 and 1300, and later became the 'default' name for Austin utes), but only as 5-door wagon and 2-door ute variants, with the same engine options as the Allegro, and like the Allegro, got a facelift in 1975.
The Allegro Mk II was produced from 1975 to 1979, and the Marina-based Okanagan from 1975-1977, when it got yet another facelift. As things stand now, the 1977 Okanagan remained in production to 1984 with the same 1.3 I4 and 1.75L I6 options. The Allegro, on the other hand, had the Mk 3 version appear in 1979 - still having both the 1.3 and 1.75 options, but adding a 1.1 L (46 bhp A-series) and a 1.5 L (75 bhp A-series) engines as options, and in 1981 adding the 1.7HL, which had the same 1.75 L A-series engine, but producing 90 bhp.
That's what's in my notes at the moment. But taking your AMC slant-4 into account, perhaps that was one of the motivations for BCRL to cooperate with AMC: getting that engine into the Allegro and the Okanagan! So in 1979, the Allegro Mk 3 is introduced (in BC, only with the 4 round headlamps), with the familiar 1.3 L I4 still available, but dropping the 6-cylinder option; instead, there are new options to be had in the 1.1 L and 1.5 L A-series I4 --- --- Now that I think about it more, I'm not sure about the 1.1 L for North America... I know there was even a 1.0 L option, but I mean... even the bitty little 1st generation Honda Civic had a bigger plant than that --- --- yeah forget it: instead, there was a new option to be had in the 1.5 L A-series... and for those wanting more brawn, there was the 2.5 L AMC slant-4, with 105 bhp will I expect comfortably get an Allegro up to 105 mph or so. As for the Okanagan, the UK's 1977 facelift is delayed in BC, appearing on the 1979 versions of the Okanagan wagon and ute; the 1.3 L is dropped, leaving the 1.75 L E-series and adding the 2.5 L AMC as the engine options. The 1.3 L Allegro is discontinued in 1982, the others in 1984.
And then, it subsequent years, the AMC slant-4 and its derivatives can replace the BL O-series engines used through the 80s and early 90s on the Austin Dogwood [Ambassador, later Montego], the Montego-based Okanagan ute, the Eagle Spirit (US versions of the Montego-based Dogwood), and available as an option on the 1984-1986 Eagle Concord [which I had initially given the O series to replace the GM Iron Duke that was the low-end option in the Concord)...
... anyways, thanks! You've given the BCRL-AMC cooperation a serious reason to exist! :)
Anyway ... a thoroughly enjoyable read, Xen! It feels believable - just enough travails and challenges to avoid the commonest ATL pitfall of everything always going your way.
Yeah, that's something I've been trying to avoid from the start, is why the DHBC story ended the way it did, with the Boeing takeover and cancellation of the Kehloke (I guess DHBC was kinda doing a BL thing, and having far too many available variations available with the Trident, instead of concentrating on one or two sizes, though there were other issues there, too, like the Merganser being rather a financial black hole...), similarly with Supermarine faltering after the success that was the Shukopoots, the Spitfire II was a bit of a misfire - but thanks to them having taken the Trident type certs on to keep the military Tridents going (and still supporing civilian Trident operators), they're still around, and there's a bit of a phoenix story in the works for them
And just a fun idea that a tiny village in the Okanagan (and its surrounding cow fields) have become a global industrial juggernaut! Hell yeah 
it definitely is fun, thinking of such things! But has a secondary benefit, too: explaining just *why* this or that particular railway line is still there and operational, or why there's much more population, etc... to end this post by tying things back to the beginning, I still need to flesh out more of why Valemount is a reasonably significant city, well, perhaps as contributing factor, somewhere along the way BCRL opened a plant of up there to supply components of some sort... or an independent company, that supplies that particular widget to various manufacturers... or maybe Hayes or Pacific moved up there from Vancouver? (Maybe both of those moved: one to Valemount, the other to one of the New Towns in the north - Meziadin, maybe, or Gitlaxt'aamiks...)
