I think you are a
Various metals and paints are used to bring me to my current form.
Buses are made of metal and paint.
At times I'm lonely and at times I become very popular.
Sometimes there are lots of people on the bus, sometimes not.
I'm very useful and you've definitely seen me at least once in your life.
Buses are definitely useful and almost everyone has seen one.
You cannot find me in your house.
That would be awkward.
Reverse the word and all the above still hold true.
A sub (submarine) is another thing you wouldn't see in your house, made of metal and paints, sometimes full sometimes not, and is useful and most have likely seen on at least in a movie/TV show.
Note that you could also make basically the same arguments for
Tram / Mart