The Trolley to Yesterday — John Bellairs

TIME TRAVELLERS
Why is Professor Childermass acting so strange? Johnny Dixon and his friend Fergie decide to sneak over to the professor's house to investigate. What they find is wilder than anything they could have imagined, for the professor leads them through a secret cellar door and onto a rickety old trolley. It's a time machine to yesterday, and soon Johnny and Fergie are off on the ride of their lives: to Constantinople during the Turkish invasion of 1453!

As the Byzantine Empire prepares for battle, the professor decides to try to save the people hiding in the Church of the Holy Wisdom. But Johnny and Fergie, desperate to stop him, have hitched a ride, recklessly unaware that they risk their lives -- and risk being lost in the past...forever!