Doomsday Cannon as direct fire TK would help
The Doomsday Cannon is on the Leviathan (which is in IG lists as well) and Colossus in addition to the Land Train. Doing this would muck all of them up. The problem is not with all of those units and lists. It's just the Land Train, so it should be dealt with on the Land Train unit.
Obviously, the quick and easy answer is to put whatever weapon stats you want on the LT and then rename the weapon appropriately.
It seems to me the real problem here is that people feel "cheated" if they are forced to choose between a small, special ability barrage and a large barrage. They are unfairly losing abilities or firepower. It seems to me there are only a couple choices on how to fix that.
1) Redesign the weapon loads across the entire train so that everything interacts more efficiently.
2) Price and playtest it as-is, with the either/or choice, and manage player expectations in another way, such as design notes, so they don't feel cheated.
"We know the barrage weapons on the Land Train often lose their special abilities. The loss of special abilities is built into the point cost. In addition, the nature of the barrage table is such that it allows quite a bit of optimization (aka min-maxing). The loss of special abilities when combining barrages partially offsets potential abuse."