That thing looks like the rails have to go in the direction you want to fire in. How useful can that be?
With the railroad guns, often tracks were laid specifically to bring the guns to bear.
The Leopold (in the image already posted) was the little cousin of the really big guns.
The German Schwerer Gustav gun was used to break the Soviet hold on Sevastopol and end resistance.

Special tracks were laid by a crew of over 3,000, and the gun positioned using those.
Forty seven rounds were fired at eight major targets in the area. cyclic rate of fire: about 1 round every 45 minutes.
Here's the wiki page, if you want to know more:
https://en.wikipedia.org/wiki/Schwerer_GustavThe shells, 800 mm in diameter (over 32 inches), are the largest ever fired out of a rifled weapon in combat.

The armor piercing round weighed 15,620 lbs. The effects were devastating, to say the least. Range was just under 22 miles. (By comparison, the Leopold railway gun shown in the image in a previous post, only had shells a bit over 11 inches in diameter)
While we might pooh-pooh the concept today, recall one of Saddam Hussein's WMD projects was a "Supergun", a 1000mm tube to allegedly be used to launch satellites, but more likely, to shell Israel.
/https://public-media.si-cdn.com/filer/1b/32/1b32c8f4-3d93-4b2b-80c2-79e23f5117c4/3538861130_695e168c73_o.jpg)
Gerald Bull, the consultant and a leading artillery expert woke up dead in the morning over this, and the gun tube segments were confiscated before they could all be shipped (the WMD that Time (and the NYT, and WaPo, etc.) forgot. Known as "Big Babylon", it was part of the Babylon Project, a series of "superguns". the Iraqi government was developing. Even the US explored the use of superguns, ostensibly to launch Low Earth Orbit Satellites.
https://www.atlasobscura.com/places/project-harp-space-gun-barbados 
and some research continues...
https://www.asiatimes.com/2019/10/article/is-us-army-supergun-just-deja-vu-all-over-again/At least the German WWII models were rail mobile.