hi Chris,
Have a look at Ray Dunakin's site (
http://www.raydunakin.com/Site/In-ko-pah_Railroad.html) < Link To http://www.raydunakin.com...h_Railroad.html) he has done an adobe building (is it adobe or concrete?) on there also if you can get a copy of the present (till Friday) version of the American Magazine Garden Railways - its £3.50.
The present one has a plan of a wood shack; the next one will paint it in a well weathered condition, that is due from the wholesaler on the 24th,
Another site is Lawrence Wallace at
http://users.stratuswave.net/~wd8jik/adobe/adobe.htm < Link To http://users.stratuswave..../adobe/adobe.htm that could help
Santa Barbara depot itself has a website at
http://www.subwaynut.com/california/amtrak/santa_barbara/index.php < Link To http://www.subwaynut.com/...anta_barbara/index.php
Hope the above helps
Your depot could be made in my favorite material 'PVC solid foam' whioch is rigid and is used to make exhibition signs which will stand up to the UK weather - use masonry paint over it, and varnish it for more protection. The masonry paint can be thickened with polyfilla to give you a slightly rougher finish: I did that on plastic corrugated board, to hide the corrugations finish of with a second coat of masonry paint etc. That is the finish on my boiler shop which has been out in the weather for a long time with no damage.
Not California style but here is a shot of my buildings taken today - they stay out in this weather!
Yours Peter.