She would ask one of the conductors: "If I asked the other conductor what train would take me to William's party, what would they say?"
Whichever train the conductor she asks replies, is NOT the correct train, so she would take the other one.
This is because if she asks the truthful conductor this question, they will truthfully say the wrong train, which is what the wrongful conductor would. If she asked the wrongful conductor that question they would incorrectly say the wrong train which is not what the truthful conductor would say.
Ultimately, no matter which conductor she asks the question to, she will get the same answer: the wrong train.
Gameworld: US103 Broadway(Express)