First: around 80-90% of trains are NON reserved-just like in GB. On these you just board and have traveldetails in app or written in diary. I untill now have paper passes and even though you are supposed to write it before boarding_this is usually the 1st thing I do when seated-to be sure that there is that day no strike/works/mishap or whatever that railways also seem to specialise in lately.

But about any newbee here wants to use exactly those trains that DO require RES and if at all possible preferably those which are the hardest to get. That includes €*-indispensible of course if you live GB and want it on the rails only.

So now the bad news: yes, this is only of the giant stumbleblocks that all those trainsaregreen promoters always omit to tell: its an incredible maze or hodgepodge  to find your way in. Even more so for only REServ compared to a full ticket.

But as also about any trip to do has already been asked and posted in this forum-a little search or read may help as 1st step. The site also has extensive country details telling you to do it where etc. And as utter baffling surprise: its also very often possible the old fashioned way-like so <2000-at the ticket windows/counters of the railways concerned.

Again to repeat: ESpana wins the prize for being worst to do, followed by the hi-speed trains in FRance and ITaly. Esp. fast trains overtheborders are hard to get and super pricey.

OTOH countries like BE - NL _DE - CZ - AT - CH (and GB for non-brits) offer a wealth of trains without any need to preplan that much

Just to add to the already helpful answer above: there is no single app from which you can make all reservations. Traveling with a pass and navigating each country's railway system does require a bit of independent research upfront. We do have plenty of information on each country's railway system on website, simply search for “Trains in X”. You can make reservations for most journeys via our reservation self-service, with the exception of a few such as Spain and Great Britain. We also have a comprehensive list of alternative reservation methods over here.