I am currently in Lucerne and will be traveling to Geneva tomorrow. I was hoping to show up at the train station tomorrow and purchase a ticket - or should I book ASAP through SBB Mobile or similar? I am hoping to travel on one of the direct trains which seem to leave hourly.
Its always nice to know you have a ticket but I believe you should be ok with those trains and schedule by purchasing tomorrow.
Showing up at the station, buying a ticket and getting on a train is how train travel is supposed to work. You will be fine. What is it you are worried about?
By default tickets are for a route, not a train, and you could even break your journey in Bern if you wanted, as normal tickets allow that. Trains are free seating and everyone gets on at the same time, regardless of when they bought their ticket.