Phil,
This is a quite interesting topic. I look forward to hearing what is to be said here by all.
Ward Kimball of course was the most publicized (and probably the first) owner of a full-scale 'backyard' railroad; but I can name a dozen off the top of my head...most of which ARE in California. If one can bear with me, keep reading:
Hal Wilmunder's "Antelope & Western" was by most definitions a 'backyard railroad'; and probably the most impressive for the time it existed. Located on his 'ranch' and I believe co-owned with his brother. Operated the Richardson Lumber Co. Heisler which the Holmes Brothers had out at Billy Jones' place, and now in Nevada City.
Henry Sorenson's "Humboldt Northern" in McKinleyville; at one time home to I believe two or three 0-4-0Ts, a Shay, and of course the Kiso #6 which, while re-gauged to 3', is once again numbered and lettered for the Kiso Forest Ry (previously bore the number 11 on its spot plate and HN lettering on its fuel bunker). Phillip Goldman in Goleta also ran the Kiso engine that's now in SLO on his "La Goleta & Shoreline" .
While standard-gauge, Chad O'Connor's "Sespe" (gift from his wife!) is worthy of mention when he ran on his property in Pasadena or Alta Loma.
San Diego area:
Charles Pollard in Vista had essentially a backyard setup, which operated as a legitimate tourist carrier (featured in the 'Steam Passenger Service Directory'). His legacy is now the Poway-Midland Railroad.
Also, Disney animator Ollie Johnston's "Deer Lake Park & Julian" (I understand his "Marie E." is now owned by John Lassetter, the Pixar producer) There's another man in Fallbrook has his little 0-4-0 in operation on his property, which made a public appearance at the San Diego Railfair years back.
For the East Coast:
Edaville to some extent was an "Estate Railroad" (I don't know the details on it) originally serving the cranberry bogs on Ellis Atwood's plantations. Turned into a tourist operation and for decades brought tourists to Carver; though there wasn't really enough of a town and nearby businesses to benefit. The Atwood estate still owned the land I believe up until fairly recently.
There was also the fellow (Morrison I believe?) in Tennessee who had the little 6 Shay out back at Roaring Camp as well as an 0-4-0T I believe, and William Willock in Long Island with a 2' setup back in the 50s (equipment now owned by Railstar; operated at 'Frontier Town' in North Hudson, NY for many years)
To close, here's Hal Wilmunder's two operable steamers operating to celebrate the completion of the 'Antelope & Western' in 1960, courtesy of Tony Johnson:
