In world war 2, germany was pretty much obsessed with Krupp railroad guns. Unlike many other things the germans loved, they were actually pretty effective, with only two guns causing 80% of artillery casualties to the allied landing in Sicily by some accounts. Your challenge is to make the Wallies use large-caliber railroad guns in some major roles at some point in WW2.
Perhaps this could be done preceding an invasion of japan, just like was planned for the little david mortar. One of the advantages of railroad guns is that when an attack is anticipated, they can be moved into bunkers with as little as 10 minutes notice, unlike other large artillery cannons.