I ran across an app called “Home Content” that I think would work for this. It’s a general home inventory app that could easily be used for model train inventory instead. It can be easily modified to add, edit or delete locations, categories, etc. search for it on the App Store. The full app is $4.99 to unlock everything. You will need the full app.
Here are two screen shots, one for LOCATIONS and one for CATEGORIES.
