It takes quite a bit of destruction-its pretty impossible to destroy them by accident but if one was really trying it is do-able.

Without a ticket they can not access their cabin (though they will wake up there each morning still). They cannot use the icp or the shop. There would be trouble if there were to be another ticket inspection.

A passenger who managed to destroy their ticket would be re-issued one on the next train platform (it would just appear when new arrivals did)