Terror on the Arkham Express

From the introduction: Arkham, Massachusetts, 1926. The party boards an express train for a three-day ride to Chicago. But strange happenings begin to occur. It turns out to be the work of cultists worshipping Yth'hawal, the elder god of darkness. The cultists must be passengers on the train, but who? Little does the party know that the train is in fact destined for Yth'hawal's realm of darkness, as a sacrifice to the dark god. Can the party identify the cultists and stop them before it's too late?