There are a limited number of 1/300th scale railway guns available. Irregular do a "generic" gun in their WW1 range and Leva do a French 32cm gun and a German K5.
The top gun is the Irregular WW1 gun, the bottom model is the Leva 32cm
The mighty Leva K5 - this is not to scale with the pics above, it's even longer than it appears.
The first question to ask about the Irregular gun is "what is it?" This is a tricky question to answer. Although the items in Irregular's WW1 range have generic names they are usually based on a real world prototype. However it doesn't appear to be true in this case. I've looked through all my railway gun books and there is nothing quite like the Irregular gun. The closest likeness I can find is the "28cm Schwere Bruno Kanone in Eisenbahnlafette." Oddly that is a WW2 German gun.
As can be observed in the picture above the model has typically Irregular crude casting. However the scanner tends to emphasise the imperfections; down on the gaming table it looks pretty good. The model comes with three crew cast on. To my mind it looked pretty bare so I added a few more. Irregulars "well fed" figures would have dwarfed the H&R crew I would usually have used so I added some Adler germans. The forward figures are Adler; those towards the back are the cast-on Irregular crew. The third Irregular crewman is hidden, he is climbing a ladder on the other side of the model.
The top Leva gun represents the "Materiel de 320mm à Glissement Modèle 1870 de 30 Calibres". It is a resin model and has very crisp detailing, you should be able to see the rivet heads. As a bonus the gun elevates. The model is not without its problems, the front just isn't correct, the rail truck should protrude further in front of the carriage, but other than that the model appears accurate.
My latest addition is the Leva K5 or "28cm Kanone 5 in Eisenbahnlafette" to give it its full title. It comes cast in crisp resin, with sharp details. The gun elevates. The model is generally accurate though it a shame they didn't include the ammo hoist on the back or the shell cradle by the breech.
The Leva models come uncrewed. I added a crews of assorted H&R artillery crewmen.
Its not surprising the Irregular gun fits the Irregular railway track. There are blocks under the wheels to stop it sliding off. What is odd is that the Leva gun also fits the Irregular track pretty well; as you can see in the photo above. It also has blocks under the wheels to prevent any lateral sliding. The Leva gun comes with a small section of resin railway. This piece of Leva railway is truly awful and can go straight in the bin; what's more the gun doesn't sit on it correctly anyway!
The pictures above and below were taken during a France 1940 game. The rails are Irregular and the trucks are Scotia.
Couple of pictures of the K5 sitting on Irregular railway