It is very much advised to book your reservations in advance. Some of them only have limited availability, such as Eurostar or the TGV Paris - Milan and can sell out weeks in advance. Also, boarding a train with mandatory reservations without having one may not be possible (Eurostar, TGVs at certain French stations) and even if you do board, it not having a mandatory reservation can lead to hefty fines or being thrown off the train.

Reservations are always train-specific.

Supplements may not be train-specific but there aren't many of those. One that comes to mind is the pass supplement for EC trains from Germany to Italy, which is only date-specific.

Thank you! I plan to book all the “reservation required” routes in advance, but I have a few routes listed (Venice - Munich for example) that require supplements, but not reservations. This is the part that concerns me! 
 

So if I prepay the supplement for May 29th but instead travel the 30th, I’ve wasted that supplement and must pay for another?

thank you,

kristin 

If you with supplements mean reservations, you should preferably book them in advance. Especially the reservations for the Eurostar between London and mainland Europe. 

The 10 EUR supplement for the Italian cross-border trains can be bought just before travelling, but if you buy the supplement at the right place you get the seat reservation for free.

Possibly. Check the conditions when trying a booking. Alternatively, this specific supplement can also be bought on the train and although they say on their website that there's an on-board extra supplement, all reports until now are that the on-board price is the same.

Thank you!! That puts my mind at ease a bit, as I was hoping to avoid paying the additional charge for the supplement on the trains. I don’t mind paying on-board as long as it isn’t an exorbitant difference. 
 

thanks so much for taking the time to answer and for your helpful input. 
 

Wishing you a lovely day,

Kristin 

For quite some routes, there are cheaper alternatives (by train). Usually by taking some regional trains, or taking a train with mandatory reservations for a shorter while. It’s a bit slower, sometimes a bit less comfort too, but it can save you quite a bit of money if you puzzle a bit.

Some are really difficult to avoid, like Eurostar. There’s no proper alternative by rail for the Channel Tunnel :)

As soon as you have a beter idea of what your route is going to look like, we can always give some advice on getting cheapest (avoiding the most hefty supplements) if you want :)

Be aware that other than Eurostar all trains in the UK (except sleepers) do not require mandatory reservations and those that offer reservations are currently free. 

I do though strongly advise you to get a reservation, but this can normally be done right up to just before departure  or online with e-mail delivery.