They have already made a lot of the trains longer, and I'd fully support them at least using the rest of the voyagers to go further with that.
The trouble is that on several of these sections, the local traffic will just expand to fill the longer trains, often with the local travellers booking seats for a short journey which prevents the seat being used for a longer journey. You need to get reasonably balanced traffic across more of the route so that you can let long distance passengers book through tickets, give them a reasonable InterCity experience, and have a sensible running-cost structure.
Ideally you also need the large crowds of people who are only on for 20-30 minutes to be able to pile in and out through wider doors, without having to negotiate their way around people with piles of luggage. That way you can tighten up station dwell times.