Who can I make a claim against if I am injured while riding on public transportation?

I was riding a Metro bus which stopped suddenly because a car turned into its path, do I make a claim against Metro, or against the other driver?

I was contacted by an adjuster from Metro/Amtrak/MARC and they said they would take care of everything, why should I get a lawyer?

I do not know why the Amtrak train derailed, how could I possibly make a claim?