Reserva de trenes sin pagar por asientos

Paris-Brussels: use bahn.com and in "mode of transport”, turn off "high-speed trains”. You can travel via Lille, using only TERs between Paris and Lille. On Saturdays and Sundays, there's a direct TER Paris-Lille. These TERs only have 2nd class.

Brussels-Brugge: no reservations possible.

Brussels-Amsterdam: use the intercity train, it runs each hour.

Amsterdam-Berlin: reservations are optional. It can get busy though and a reservation is only €3 on tickets.oebb.at.

In Switzerland, no reservations are needed.

In Italy, you can use regional trains to travel without reservations. Use trenitalia.com and change the search filter from "Main solutions” to "Regional”. No 1st class available though and it takes a lot longer. There are often also intercity trains, for which the reservation is only €3.

Madrid-Barcelona: there is 1 daily regional train without reservations, but it takes 9 hours instead of 2.5-3 hours by high speed train, so the €10 reservation may be worth paying for.