@Camilo 

In the event you cannot find an upgrade route through the reservation company you used, I would simply board the train and take any empty first class seat. If full I would simply take my reserved second class seat and put it down to experience.

Except in a very few cases, and I think of course this €* that all the Brits moan about-train staff will not be happy with it-there may be substantial higher surcharge for 1st vs. 2nd. But indeed in many other cases this is good advice. ALWAYS think and give full info for better answers

@mcadvSo you think @Yorkie is right?

Where we only have a 2nd class seat reservation, you think we should take a 1st class seat?

It seems something has gone wrong. I know the Interrail system has glitches, as we’ve been emailed about a couple of glitches when making reservations. Is it possible that that first class seats weren’t available when we were booking and now are available? Or that when we put the booking through that we only standard / second class seats when we booked, but we should have received standard premier / first class seats?

Something seems very strange, that’s all.

In fact, I’ve just noticed that on our Eurostar inbound journey that our reservation is only for Standard, and it states “Holder of valid Eurail/Interrail Pass Second Class only.” But as I say, we have a Eurail/Interrail First Class pass.

There is no problem using your first class for a second class reservation. You can always check at your boarding point whether E* has any 1st class availability and how much to upgrade. Interrail 1st class E* is about £10 more than 2nd).

As my previous post (where I hadn’t registered it was E*), worst case scenario is you take your standard seat and put it down to experience.

@Yorkie Thanks for the response. It’s not only Eurostar. It’s happened on SNCB and SNCF. The price for the SNCB first class reservation and Eurostar are such a relatively minor difference that I don’t think we’ve made a mistake, I’m concerned something has gone wrong in the booking system.  I’ve sent an email to support, though, so hoping this can be resolved.

It might be useful to point out that this can also be due to availability. If first class pass holder seats are fully booked, then our system does automatically seat you wherever there is still space available. In the case of Italian trains, there is sometimes a systems miscommunication where travelers are assigned seats in the wrong class. you can simply have this rectified at the station's ticketing office.