Lucy must ask one of the conductors...

"Which train would the other guard tell me I should take?"

Lucy should then take the other train.

If the conductor she asks is the one who lies, then he will lead her astray by telling her (falsely) that the other conductor – the truthful conductor – will advise her to take the second train. This makes the first train the one that Lucy needs to get.

If the conductor she asks is the one who tells the truth, then he will tell her honestly that the other conductor – the lying conductor –will advise her to take the second train. This makes the first train the one that Lucy needs to get.

Conversely...

If the conductor she asks is the one who lies, then he will lead her astray by telling her (falsely) that the other conductor – the truthful conductor – will advise her to take the first train. This makes the second train the one that Lucy needs to get.

If the conductor she asks is the one who tells the truth, then he will tell her honestly that the other conductor – the lying conductor – will advise her to take the first train. This makes the second train the one that Lucy needs to get.

This can be interpreted more simply as a yes/no question, by asking either conductor, “Would the other conductor tell me that this is the train that I need to take ?”

The truthful conductor would say No if it was the right train, because the other conductor would lie about it, but the Lying conductor would also say No, because the other conductor would truthfully say that it was the right train.

If it wasn't the right train, then the truthful conductor would answer Yes, because the other would lie and say that it was, where the lying conductor would say Yes, because the other would truthfully tell her it was the other train.

Server, Loch Ness, unless Cylinder Head (Express) server is running.