If trying to imagine a point of view from early 20th century, when you had to face strong fortifications like Verdun, Maginot, Siegfried or Stalin Line, I could sure see why they wanted some very heavy guns - and there really wasn't any alternative than rail to transport heavy things over land.
If it had been necessary to assault say the Maginot Line those very heavy and very expensive monsters might have come to good use. But you would also have years in advance to lay track lines and lots of them.
Forget about turrets for guns larger than 6-8" - the recoil simply is too powerful, and having a 50-100 ton barrel point sideways of the track will seriously risk having the mount tip in a breeze.
Next forget about having heavy rail guns be of use relative to their cost in anything resembling moving war, the rail lines are probably not in the right place anyway and will be needed for logistics.
But if we recognise the need of a weapon system to take out very heavy fortification you might meet on your advance I would rather consider a very heavy mortar. Have it mounted on a heavy tracked vehicle (heavy tank size) and before firing you "tip" it down on a baseplate on the ground and with the vehicle functioning as the bipods of a traditional field mortar. The recoil would be taken by the ground but could be supplemented by recoil dampeners. I guess it would be possible to have calibres of at least 12 " perhaps much bigger.
Such a weapon would not range very far, less than 10 km, but could get into and out of firing position very fast and so be less vulnerable to counter battery fire and air attack. The shell would hit at a very steep angle and so be ideal for penetrating deep. I imagine it being loaded in the hiding position, then move forward and fire and back again to reload. With a team to provide data on weather and position I think it would be possible to hit very precisely in first shot at such a limited range.
If an air force guy I would suggest a wire guided bomb. Imagine a bomber with half its payload being a bomb and the other half a huge cable drum. I have no idea how much strong enough cable you can get for a ton, but the idea is to fly over the target, drop a bomb with controllable fins and drawing a cable after it through which you send steering commands. Also put a flare in the tail of the bomb, so the aimer can see it. With a little practice I guess you could be very precise with such a gadget and it would be very difficult to "jam". In OTL the allies relatively fast found out how to jam the German radio controlled anti-ship bombs.
If Japanese I would mount a saddle and some harness on the bomb and connect it to steerable fins and put an advert in Tokyo Daily: "Cowboy type needed for glorious job!" (with due inspiration from Dr. Strangelove).