Can I ask another passenger if she'll say I'm part of her family for the train ride?
That's strange. Why would you want her to say you're a part of her family?
I can't afford the train cost so I'm hoping I can ride for free.
I know you can't afford the train ride, but it's wrong to pretend to be part of someone's family to ride for free. You have to budget your money better so that you can afford transportation costs when you need to take the train.
Why? Everyone's looking for a free ride.
Because it is wrong to try to scam a business by pretending to be something you are not. You could get banned from using the business in the future.