It is HO and it is the club layout of the La Mesa Model Railroad Club, within the San Diego Model Railroad Museum building in Balboa Park, San Diego. One of the finest model railroad exhibits in the nation.
There is also the San Diego Model Railroad Club, the San Diego N Scale Society (both of whom model Carizzo Gorge in HO and N scales, respectively). There is a 3-rail O tinplate layout there also as well as the San Diego Club's O scale layout.
Quite a complex but the La Mesa Club is the shining star, IMHO. They are modeling Bakersfield to Mojave complete with a fully functioning CTC system and as close to scale in size and scope as possible. The stretch from Ilmon to Cliff is fully sceniced and the Rowen through Marcel area, including Tehachapi Loop is finished for trackwork and awaiting scenery. There is about 10 feet of separation in elevation between Bakersfield yard and Walong. Just an incredible layout.
None of the equipment seen here is mine, I was just privileged to watch it run!

So I cheated. Not really my models but since we were running comparisons of model vs. simulator and I was running this thread
Grand Canyon on Tehachapi I thought it might play.