"And you're absolutely sure you're the only person around who could figure it out?"

Because the people on the train do have varied talents.

And Alice doesn't doubt that for at least one of the people here, some of said talents involve poisoning people and preparing said poisons.

It's not even that she doesn't trust Alleyana - the other has seemed reasonable enough so far to have thought this through. But some things are harder to take into account when you're not used to the wild, wild people that occupy this train.