As well as ministers moving on, so do civil servants. In the years between a decision being made and the resulting outcome of that decision being known, there may well be nobody still doing the same job.
This is where the HM Treasury '
Magenta Book' should kick in. Decisions result from processes and the decision on say rolling stock will be based on trying to fulfil dozens of policies, within many constraints, with limited data and a very large dose of uncertainty about the future. Hindsight is easy, foresight is really difficult. If someone does manage to predict the future, it's more likely to be luck than skill.