There is a book by Charles S. Small called "California's Railway Guns". It is a 90 page paperback. There are drawings and photos. The photo reproduction is not great.
A gun similar to the one in the second photo is shown on the cover of the book with its railway trucks. There was a span bolster with two six-wheel trucks on the breech end, and a span bolster with two eight-wheel trucks on the other end.
It (the book, not the gun) is available from Amazon for about $1000 (plus $100 shipping). And elsewhere, for substantially less.