Nice looking model, but I would like to point out a couple of things you might change. The ammunition should be in separate pieces, not put together. First the projectile was loaded, then the powder charge / charges and finally the casing. They would never be found stuck together as one piece. Here is a video that shows German crews loading and firing various heavy guns, including the K5;
https://www.youtube.com/watch?v=oxykqgRodTc
Here is a link which describes the types of projectiles used and the three different sized powder charges;
http://www.one35th.com/model/k5/k5_ammunition.htm
The other minor change I would make is to have the weapon being towed from the other end. This avoids the clearance problem between the cab of the locomotive and the end of the barrel when it is secured in it's travel-lock. This arrangement can be seen in the video.
In this case, "Robert", one of two K5(e) stationed in the Anzio area, a small boxcar was added between the locomotive and the end of the gun, thus avoiding this clearance problem. The roof of the boxcar then provided a convenient platform from which the crew could clean the barrel.
Judging from the unevenly cut vertical supports, simple roof and stake-bed sides, it appears this "boxcar" was field converted from a gondola car, not a standardized production type of rail car.