What is a main station in Europe? As to where you can buy seat reservations

In the good old times you could book in London or Amsterdam a ticket till Russia and even further but with the digitalisation all changed and each Railcompany use a own system :/
Because of that no regular ticket office in the UK can reserve trains outside of the UK (few specialised ticket agency´s still can do it )
Amsterdam
they are able to reserve more but how much is quite difficult to say its a try and error :/


Spain and Portugal made it quite hard to get reservations from abroad.

That depends on where and with what type of train you plan to travel. If you share your travel plans here you can get advice on if there are reservation that you can make in advance. 

That seems really nice. 

Are you planning to make as many reservations as possible in advance or only to make reservations for the trains where reservations are mandatory, like high speed trains (not in Germany) and night trains?

There are many train where no reservation is required and in some countries reservations are rare. The Eurail planner is not always up to date on the need of a mandatory reservation, or even the possibility to make reservations, so the advice is to check the website of the railway company that runs the train to get the accurate information. 

As an example, in Switzerland, only a few scenic tourist trains require reservations. On the routes for these trains there are also regional trains without mandatory reservation.

I will be Interrailing with my daughter this summer in Germany, the Czech Republic and Switzerland. The only reservations that we will have is the night train between Sweden and Germany.