How to reserve seats if I am "on the go" and do not have access to a printer? Can I get a QR code sent to my phone?

Depends on where/when >But MOST railways now send it as mail and if its optional you generally do not even need to show it or will be asked for. IF its mandatory (like FR TGV) it also all can go on the fone, maybe you have to download yet antoher app for that-every railway is different.

Alternatively, you buy reservations from a ticket office.

Thanks to those who replied to my questions!  Enjoying a wonderful European a river Cruise at the moment and will check with rail station Before our first journey.

thanks again!

Ken