When used in the past tenseA grammatical category that is marked by verb inflection. To express when an event or action happens in time or when a state exists, we use tenses., this structureThe way in which the parts of a sentence, clause or expression are arranged. E.g.: make somebody + infinitive in "The teacher made me rewrite the composition." refers to an actionA general term expressing that somebody or something does something in the past which was fixed to happen at a determined timeA concept which is related to our perception of reality; that part of existence which is measured in seconds, minutes, hours, days, weeks, etc., and which may or may not have happened.

The train was due to arrive in a few minutes.

Modals expressing past necessity / obligation
Modals expressing past necessity / obligation BE TO + infinitive