The so called “Luzern Interlaken Express” is a normal service train that happens to have big windows and operates along a scenic route. It has only recently gotten the “PE” moniker so that tourists can be tricked in to paying for an expensive reservation (which means more money for the operator) that is not mandatory at all on this route.
If this train were to not permit luggage it would be useless. People use that to actually just go places. It is not there just for tourists.