Showing The Trolley to Yesterday by John Bellairs

Johnny and Fergie are worried. Professor Childermass has been acting weird. Very weird. In this sixth Johnny Dixon mystery, the two boys sneak over to the professor’s house to investigate, and what they find in the process is more amazing than usual--a talking statue, the ghosts of long-dead Crusaders and a rickety old trolley that turns out to be a time machine.

Soon, our three heroes have traveled back to Constantinople in 1453. The Turks are invading and Professor Childermass has taken it upon himself to save the people trapped in the Church of Holy Wisdom. Johnny and Fergie must stop their friend from getting himself killed on his noble but doomed mission but, in doing so, do they risk their own lives? Will they all be lost in the past forever?