As Bruce points out, the track gauge is measured from the inside edges of the rails.
If your track gauge as measured above is exactly 1/2-inch, that's a ratio of 1:117. That is much too small for HO and is pretty close to TT-scale which is 1:120.
From your measurements of your engine and cars, they are also smaller than HO and perhaps close to TT-scale as well. So I would say that TT-scale buildings -- if you can find them -- would be about the right size to match your Power Trains.
My recommendation: If you like the Power Trains, set up a layout and watch the trains go round and round. That's a perfectly acceptable way to have fun! No one should criticize you for doing that. You can even get the crane set and load and unload some cars. If you can find some TT-scale buildings, so much the better. - Okay...Since you are suggesting TT-scale buildings, my next question is this-Are there any web sites or hobby shops you know of which will sell ready-made or easy-assembly TT-scale buildings? If so, how much will they cost?
The harsh reality is that the Power Trains engines, cars, and track are not realistic in either appearance or operation compared to the products offered in the established model railroad scales. Power Trains has not even attempted to create accurate scale models, but has instead made what I would call extremely rough approximations of generic railroad equipment.
Just from looking at photos on the Power Trains web site, it appears to me that the engines and the cars are out of proportion with one another. Either the cars are too long or the engines are too short -- not sure which.
The only thing that having an actual HO engine, car, or piece of track would do for you would be to show you how much more realistic it is compared to any of the Power Trains products. You can also do that by taking your Power Trains engine into a local hobby shop and comparing it to the HO products on display.
- Jeff