If about is still a preposition, where is its object?
The object is the proposition - whatever comes after "How about". "How about if we go by train?" "How (do you feel) about the proposition that we go by train".
"How about" is a collocation
I would say "How about we go by train". The "if" isn't necessary here, and it's probably ungrammatical. But that's the way some people say it, and the "How about" part means the same thing.
"How about I kiss you?"
(What would you say to the proposition that I kiss you? How would you feel about my kissing you?)
"What about if he's escaped already!"
(What are the consequences of his already having escaped? What should we do if he has already escaped?)
The proposition can be anything; and "How about" or "What about" simply means "consider the case of/about".