Are you planning in Italy? Trenitalia have just published their summer timetable (starting 12 June) for their long distance trains (not yet for their regional trains though). The rail planner app hasn't had an update since 3 March so it is becoming outdated and certainly does not have the Italian summer timetables yet. This is also why you shouldn't use the rail planner app to plan. I suppose it will be updated again soon though, with hopefully the Italian summer timetable.

Generally, it's best to use national websites or apps to plan and for international planning the Deutsche Bahn planner (updated twice/week) or DB Navigator app. I don't have much experience with Trainline so I can't say much about it. For Italy, www.trenitalia.com is the best source.

Reservations can be booked via Interrail (booking fee: 2€ per person and train) and in many other places. For Italy, ÖBB is recommended (with these steps; no booking fee).

Trains are not planes and your worries turn up here very often-daily in fact, but have no ground. In fact in many countries the holidays are the slower time for trains-no students etc. In the not even far past it was customary for trains-IF a REServ was needed, to allow this only 2 month advance, by now its mostly 3 or sometimes 4. But every country has its very own railwaysystem with the needed extensive burocracy to go with all govmt-institutions on how to handle all that. Just scroll down and read-I bet that 90% of all your potential Questions have already been asked&answered.

There is one other snag: early june many railways start their summer timings (more imprt is the main change in dec for next yr) and often these are only decided very last minute.

The major prob for me seems to be that each&every newbee wants to use the very same cumbersome trains in FR and ES that require expensive extra REServations and supplmts. Best advice is to change plans and go slow!

Best site for general planning is NOT that EUrail app, but bahn.de from Germany, or often indeed thetrainline. THere is NO specific site that will show if a train is in/out of pass-use, except that of coruse the app will only show trains IN-but does not list them all-in fact none in some countries or just a very few in others. Also very good for tipical brit-inclined train users is seat61. OR-who would have thought of that-the general country info in the main site. EU/INterRail-with links to the national planners etc and which trains do/not accept pass. Except for a very few countries you can expect that the pass is accepted on all trains on nearly all major lines anywhere in EUR. Tipically those from UK tend to think in companies-but here in the continent its often different. An INTern. train may even have cars from 3 or 4 different ´companies´ and by magic a german train becomes french after crossing border!