May 31, 2018 - 11:27 PM
In 56 years, I've never been unable to board a Swiss train because it was too full. It doesn't really happen as the trains are frequent and Switzerland is a small country and cities are small.
Zurich itself is roughly 1/16th the size of London for example.
Furthermore, Luzern is a terminus station so just get on the train as soon as it is in the platform and there will almost certainly be plenty of seats available.