Reservation Zermatt to Montreux via Visp

Wondering is seat reservation is required while travelling from Zermatt to Montreux via Visp? no option on seat reservation is currently shown on SBB website.

As far as I can see, all connections between Zermatt and Montreux are on Regio and InterRegio trains, on which reservation is not possible. Reservations are the exception, rather than the rule, in Switzerland.

Alpenrose