The US converted several coast defense 12" mortars to RR mountings for use in WWI. A photo in the London Daily Mail of 1914 of an Austrian 12" mortar caught my eye and the search was on. Here are the results -

12 inch US railway mortar

https://en.wikipedia.org/wiki/12-inch_coast_defense_mortar

US 12 inch coast defense mortars mounted on railway trucks for use in coast defense in the late 1800s and early 1900s.

13 pix of 12 inch US RR Mortar

(3 of these are in Railwaygun File section at https://groups.io/g/railwaygun/files/12 inch mortars)

https://commons.wikimedia.org/wiki/Category:12_inch_railway_mortar

+ 50 pix (not RR mounted) = https://commons.wikimedia.org/wiki/Category:12-inch_coast_defense_mortar

film clip of firing,= https://www.youtube.com/watch?v=poAApHtjE-A (This has disappearing gun footage)

plotting room in action = https://www.youtube.com/watch?v=YxmF6sEmzVU

Austrian 1907 12” field artillery mortar

A) https://www.pinterest.com/pin/129267451777038069/?lp=true ,

B) Cannons of WWI - Skodamörser 305mm

100+ pix & drawings = http://www.moesslang.net/kanonen305mm-skoda.htm 

diorama = Pinterest 30.5cm Mortar diorama link =https://tinyurl.com/y27f6a9w

Full url = (https://www.google.com/search?q=Daimler+M17+Radschlepper&client=firefox-b-1-d&tbm=isch&source=iu&ictx=1&fir=rgjklLgzgwtwpM%3A%2CJLyCKiuP6yBnOM%2C_&vet=1&usg=AI4_-kSWcSoXpMEoEe3-4IPzg7dkZ3Drxg&sa=X&ved=2ahUKEwjp2uK1kvHhAhWEdt8KHYYQDVAQ9QEwBHoECAkQDA#imgrc=BSD0lf-8w0_MRM:&vet=1 )

The wiki article is comprehensive!

Phil, Bklyn