What do you mean that there's an empty carriage at the back? That's no good, we need an extra one at the front!
But an extra carriage at the front will mean that those passengers who regularly cut it too fine will miss the train, - better at the back making it easier for them.

Seriously though, assuming you mean what the interviewee meant. I don't know but it is an oft quoted comment from random members of the public and blindly repeated by the lay press who seem to think that adding carriage(s) is a simple option whenever trains get overcrowded. That 'ship sailed' way back in the last century on virtually all NR routes in the UK.
Of course, it's sometimes possible to attach an extra Multiple Unit to cover exceptional peak demands provided the stations stopped at can accommodate the additional length, but fleets are sized for the planned services and timetables crafted with the most economical diagramming using the available stock.
However, it appears that tacking a 2-car DMU on the end of a 2-car service is frequently done up north, if there are any working ones available!