Just saw some replies were added after may...
I have the answer to my question by now. I mailed the railroad company and they told me:
The only difference is that there are fewer trains on the rail (one vs up to five before 4:30 pm) and so the train can travel just a bit faster and the turnaround time is less.
So we're happy to keep our 4.30 PM departure time and are looking forward to the trip.