EC 276 Train from Budapest to Prague July 2019

We too took that route 3 years ago. We also found the trains run down with very ineffective air conditioning even in April. And we were in a compartment with a bunch of construction workers on the way home for the weekend--with one trying to sleep on my shoulder. And the train was slow and made many stops.
These countries' train systems have not yet joined the modern world.

I've been bumped from my seat by a those holding a seat reservation for that seat in 1st and 2nd class. It's expected you give up the seat. How often does this happen? Not very, but still considered train etiquette in Europe.

When that seat reservation is shown to me, I know it's time to get up and leave. Riding EC trains I always ride 2nd class. In Czechia, Poland and Hungary ie, Budapest to Vienna, I never ride 1st class.

if there was open seating in second class why didn't you move to second class instead of standing?

The train was packed like a sardine can. There was no room to move around and look for seats. people were standing in every nook and cranny on the train.

"The train was packed like a sardine can" How often have I been on regional trains in Germany and France exactly as you described but not on an EC train.

A couple of years ago on the ICE train Leipzig to Berlin, I had to, along with everyone else, stand the entire ride in the area at the end of the coach, the area of the WC, where every one was packed in. All locals, everyone just put up with the crowded condition, everyone stayed civil too.

I agree that Seat61 is a valuable resource. It is ironic that I had done a lot of research prior to purchasing these tickets, including consulting Seat61. I don't remember declining the option to purchase a seat assignment when I ordered the tickets. When I noticed that my ticket did not include an assigned seat, I emailed customer service and asked them if I could purchase a seat assignment. They did not give me that option, but instead suggested it was unlikely that the train would be full.

These communications were made more difficulty because I don't speak Czech and the customer service rep spoke limited English. The train breaking down was bad luck. Stuff like that happens.

Despite the crowding, most everyone was very civil and accepting of their fate. My intent in creating this thread was to publicize that purchasing a ticket and purchasing a seat are independent of one another. A purchased ticket does not guarantee a seat. I learned a valuable lesson, and, hopefully, wont fall into the trap of having a ticket but not a seat again.

Keep in mind that for certain trains seat reservations are not sold. No seat reservation for TER trains n France, and the RE and RB trains In Germany.

The seat reservations are sold for EC, ICE and IC in Germany, and RJ in Austria, and are mandatory for the TGV in France and the Thalys trains in Belgium