You can always buy a seat reservation if there are seats available on a train. The ‘Interrail allocation’ only applies to the cheaper €10 TGV reservations; once they’re sold out, the €20 reservations offer last-seat availability.

That said, August is a busy month with the Olympics and it wouldn’t be surprising for trains to be entirely sold out.

I think your best fallback option will be to take TERs to reach Geneva, as you say, and then use the Swiss and German railways (no reservations needed on most of those).

The Spanish AVE, which travels from Barcelona via Perpignan and the high speed line to Lyon/ Paris, will I believe let you board without reservations, subject to there being space and paying €10 to the guard. I believe they’ll let you travel in the bar car if there are no seats, so that could be worth knowing about.

Routes to Paris should be fine as they're quite frequent, especially on evenings and outside weekends.

However it probably wouldn't work for international trains, Eurostar in particular (passholder quota on these). Neither for TGV intersecteurs (those avoiding Paris) : they're very popular.

You'll likely find something but it'd require a bit of flexibility. As long as you don't have to take a particular train, it should work!

Do not hesitate to ask for advice here closer to travel date.

I find the French TERs are usually very comfortable (there are exceptions). Depending on the material, easily most comfortable regional trains in Europe. In those regions it's mostly AGC trains or Régiolis, which have seats that are on par with IC trains  in some countries in Europe.

Slower, but comfortable should the TGVs be full. 

You can't take 2 night trains on one travel day. A travel day is not any 24 hours, but 00.00-23.59 CET.

Of course this is possible. You may go from Denmark even to Palermo with one travel day taking two night trains. 00.10 EuroNight from en København Airport, possible to be in Milano at 19.17, 20.10 night train to Palermo, no new travel day needed. And this will work with many connections from Denmark.  

Thank you for your answers

It is good to hear that it is possible to book reservations as log as there are free seats.

I can find the Spanish AVE from Barcelona to Lyon. But no Renfe-train all the way to Paris. Not that I need to go to Paris, but am I missing a route?

Can you tell me more (link) about the possibility to with AVE i France without a reservation? Is it a real rule or do I have to be lucky to meet a guard in a good mood?

Should I ask the guard at the platform (the train only stops for a few minutes) or should I just get on and ask after the train has left the station. I can’t risk being left behind if I have reservations further north.

I assume there is no Renfe staff at French stations.

It is also good to know that TER-trains should be comfortable enough for long journeys. I was worried that the seats would be hard and narrow like some commuter trains.

A bit off topic, but i case someone is wondering how I can take 2 night trains within 1 travel day, here is my outbound itinerary:

Horsens 23:14 to Odense 00:23 with DSB IC 1480 (regular ticket)
Odense 01:46 to Hamburg 06:00 with SJ EN 345 (I found this departure in DBs planner, but there are some indications that this train won’t stop in Odense. I can’t make a test-booking at SJs site. But DSB International says in an e-mail that they can sell me a reservation. If it fails I can either get on the train at Copenhagen Airport or use DSBs seat-only train that runs almost the same time as EN 345)

Hamburg 07:23 to Mannheim 12:27 with DB ICE 595

Mannheim 13:42 to Paris Est 16:52 with TGV 9552

Paris Est to Paris Austerlitz with Metro (Not covered by Interrrail)

Paris Austerlitz 22:11 to Pau 08:41 with SNCF D 3131 (No time schedule yet, so it might change number and time a little)

Pau 09:20 to Bedous 10:26 with TER (Regular ticket. No time schedule yet)

Bedous to Canfranc with bus (Regular ticket. No time schedule yet)

I have made extra time between the trains in Germany WHEN (not IF) they are delayed. I am even considering to buy refundable reservations for 3 different trains between SW Germany and Paris. The last one arrives at Paris Est 20:43. I will then have to get to Paris Austerlitz before 22:11. I don’t want to miss this night train. It will be almost impossible to find a place to sleep in Paris just before the Olympics.

Palle Dam 

There is no AVE (yet) all the way to Paris. There are a few TGVs though.

To travel from Barcelona to France without reservations, use the DB planner and unselect "high speed trains” and "intercity and eurocity trains”.

RENFE doesn’t run (yet) to Paris : the initial plan was to start this summer but the French bureaucracy didn’t agree… it’’ll more likely be at the end of the year or in 2025.

Speak to staff on the platform before boarding, otherwise you’ll likely get a fine (it’s treated as travelling without a valid ticket). Reservations are really sold on the platform : staff are used to it and plenty of people used that option, including a few people here I think. I wouldn’t necessarily risk it in summer though, especially if you’re not boarding from the departure station.

Better to book a TGV a day or two in advance.

Of course if you’re in Narbonne and only need to go to Lyon or Geneva that day I’d do it : there are plenty of slower regional trains in case. But if you must take another TGV followed by a night train, don’t do it.

About the journey towards France now :

In addition to the SJ night train (I’m not sure you can board it in Odense honestly) there is an overnight EC in summer :

• Copenhagen 23:59
• Odense 01:24
• Hamburg Hbf 06:27

Seating carriages only but it’ll be easier to book.

An hour margin in Mannheim should be plenty enough, I’ve been regularly delayed 20-30 min delayed but rarely more than that honestly. Stats for ICE595* also show that the train was more than 45 min late only twice in the last month, the biggest delay is also both times near Munich (its terminus).

*ICE595 currently does Berlin - Mannheim - Munich as the Riedbahn is still open. From mid-July to mid-December all trains between Frankfurt and Mannheim and thus departure/terminus stations, train numbers, times, everything will change.

ICE73 (the one currently leaving Hamburg at 07:23) was only once delayed more than 30 min (by 36 min).

All that to say : I wouldn’t bother with booking extra reservations. 1h15 is enough margin, even for German standards. You’re also entitled to travel on the next train if it wasn’t sufficient for some reason. If all later trains from Mannheim are sold out, whether in 1st or 2nd (quite unlikely) you can always travel via Offenburg - Strasbourg. But you won’t need to go that far.