It will depend on the country and train, In most places there are no boarding controls so it may be possible to board but on reservation compulsory trains in countries where non-reservation travel is not allowed you may be ejected at an intermediate station or even face a legal demand for money from train officials.

Tell us where and when you want to travel and you may get a more useful response.

At least in Sweden that is not recommended to board a train without reservation if the train has mandatory reservation. You will have to leave the train if you are caught. 

Thank you very much. I want to go from Italy to Prague, but every train seems to be full. I’d choose this one:

Do you think it is too hard to board on these trains that require seats reservation without the reservation?

Take 16.43 Milan Central - Lugano 18.04

and

18.30 Lugano - Zurich 20.27

or

19.01 Lugano - Zurich 20.55

none of which require reservations and give a better margin for delays than a 13 min connection in Zurich.

The 21.40 Zurich - Prague is bookable on OBB site but it shows it as being a rail replacement bus at 06.52 from Linz - Donau with rail from there to Prague.

https://tickets.oebb.at/en

OVernight trains ALWAYS must be reserved-also for seat-only and can easily be done via oebb.at (use full ticket-then choose discount as pass- NOT use ´seat only´ option)-price is 14/15€ for seat. These replacem. buses are free to use-they probably dump you at some spot where the rails are in order agin for onward.

I am travelling en route from DK to Barcelona and the last leg is Lyon part Dieu to Barcelona 1430.

there has been no way of ordering a seat over the net, tries everything, and not even through my national train service.

I am counting on being able to book a seat on my senior interrail pass, in Lyon itself as I arrive, but now I am not so sure…

Checking Renfe on this hi-speed train says seat reservations are only to be had in Spain itself and in person..

Calling Renfe in Spain, they will not guarantee that the possibility exists of actually booking a seat in Lyon…

Reports says on the net ( and on this forum) that this glitch has been there for years, but it is my only option, once my itinerary has been entered into my pass, and this route was the last option here.

And there is a booked room in Brcthat very evening, so….

the train is  :

AVI 9742 

AVE International      5/11    1430 to 19.30

Can I feign ignorance and just board and hope for the gift of the gab to kick in….

Don't board without a reservation. You must speak to the train crew before boarding and ensure that you can buy a reservation on board. Otherwise you risk getting a heavy fine.

you are right, I would use the innocence of ignorance to convey that this could not possibly present a problem for the railway !

thnx