One thing that will never stop surprising me is how long-distance train journey are so much more convenient and cheaper in Russia with its crap rail infrastructure that was barely touched in the last 50 years, compared to EU with its "high-speed" railways.

Now high-speed rail sucks in Russia. When they introduced the first (and the only so far) high-speed train in Russia, to connect two largest cities (metro populations: 15 million and 5 million, 600km apart), everybody laughed at this, because while all the other countries were busy building their 300+km/h railways and 300+km/h rolling stock, in Russia they just purchased a Siemens train qualified for 250km/h max, and only built a tiny piece of new track for 200km/h operation, while the rest remained the same old track (very good, the best in Russia at its allowed speeds of up to 160km/h, yet with totally inadequate capacity, and of course 160km/h is laughable when other countries connect less major cities at 300km/h).

Yet RUssia has an extensive infrastructure of sleeper trains.
The trains are very aged, and the railroads mostly suck (a typical railroad is maybe qualified for 100km/h operation, and maybe was last updated in USSR times), and the schedules suck too, and because of the different voltages in different regions, 30-minute or 1-hour-long scheduled stops for locomotive change are very common, and also stops at large cities are often also half an hour long (probably also to provide some padding for eventual delays), and through ticketing is non-existent (if your journey involves any transfer, you have to buy separate tickets for separate trains).
However, they run literally everywhere where there is a railroad, any two major cities or towns that are reasonably connected by railroad, are connected by direct sleeper trains, maybe it's going to only be once per day or once every second day, maybe it's going to only be a single car attached to several different trains on its route, but still, most likely you won't need any transfers.
And that's in country that's 4x larger than EU while having 1/3rd of the population!

For example, Yekaterinburg is 1400km away from Moscow as the crow flies. There are around 10 direct trains per day, the fastest taking 24 hours (with the average speed of around 70km/h), median travel time is 27-30 hours (with the average speed of 50km/h), which is laughable when compared to HSR speeds of 250-320km/h. They do not have seats; there are only open-plan couchettes, couchette compartments (four couchettes per compartment; prices start at 50 euro per couchette for the entire journey when bought 2 months in advance, up to 100+ euro for popular trains close to the departure date), and sleeper compartments (two beds per compartment; usually 2x the couchette price, i.e. the entire sleeper compartment for two costs the same as the entire couchette compartment for four, from 200 euro when bought in advance to rarely 300-400 euro when bought at the last minute for popular trains), and these tickets are mostly flexible (refundable in full for a small fee of a couple euros before the day of departure, and partially refundable on the day of departure or, very partially, even up to one hour after the departure). The trains are typically very aged (although I heard this improved recently), with toilets of a kind that sends the waste directly to the track below being most common kind just 15 years ago, and of course no luxuries like washbasins in sleeper compartments. Yet they get you from point A to point B. And these ten trains between Moscow and Yekaterinburg travel along two different paths (one via Kazan, another via Kirov), and they continue onward from Yekaterinburg to all kinds of places, including Novy Urengoy and Vladivostok, with stops in all the major cities / towns along the way, plus some of the smaller stations (with stops typically once every 50-100km).

We're going from Berlin to Bordeaux this summer. I heard there were night trains in Europe before, but I don't know how they've been. What I do know is that Berlin is slightly closer to Bordeaux than Moscow is to Yekaterinburg, and that it takes 16 hours and three interchanges on "high-speed trains" to get there, and that such a route only works once per day and costs over 100 euro for _seats_ and requires getting separate absolutely non-refundable tickets from separate companies meaning that one train delay might cause the entire journey to fall apart and will likely force the passenger to buy a new ticket out of pocket.
These are "high-speed trains" yet the average speed on this journey, all interchanges included, is under 100km/h.
If this was Russia, there would be no high-speed trains at all, the rail infrastructure would be extremely dated but there would be plenty direct trains like Sczechin - Berlin - Stuttgart - Lyon - Bordeaux or Warsaw - Berlin - Frankfurt - Paris - Bordeaux - Bilbao with stops every 50-100km and average speeds 50-70km/h, taking people from Berlin to Bordeaux in several hours more than the current possible journey plans but without any interchanges and in the couchette at half the current price of the seat.

Similarly, in order to get from Berlin to Rome (similar length of the journey), one would need to take three or four trains on separate tickets, and the connection _really_ does not work without an overnight stay somewhere, so that's again over 100 euro for a seat (when booked in advance, non-refundable), plus a hotel stay, and over 24 hours total. Even without an overnight stay, the fastest (but not guaranteed that you won't miss the connecting train) connections seem to be around 20 hours. Again, these are all "high-speed" trains classified as ICE in Germany, running on high-speed rail!
If this was Russia, there would be crappy aged trains on crappy aged railroad, but they would be several of them per day, on the routes such as Stralsund - Berlin - Basel - Milan - Rome - Bari and Aarhus - Berlin - Innsbruck - Rome - Syrakuse, with plenty of stops along the way, and taking just a couple of hours more than a "high-speed" connection and costing half of the Sparpreis seat price for refundable couchette ticket.

(And that's on ancient infrastructure without almost any investments! If EU approached this the same way China did, with its comparable area and population, then there probably would be a direct Berlin-Rome or Berlin-Bordeaux high-speed train with seats, only taking five hours. I don't know, I'm not an expert on China.)

How is it possible to invest so much into the railways, into all that HSR hype, and still end up with something just as slow as was typical in Russia 40 years ago ffs, but so much more expensive and inconvenient?

(Yes, I know, there are Nightjet trains. Have you seen their route map or their prices? 100+ euro for a non-refundable couchette, or 300 euro for an entire sleeper compartment, from Vienna to Genoa (mere 700km, yet taking 14 hours)? Unbelievable, these trains are slower than the typical ones in Russia, yet 3-4x more expensive for a non-refundable ticket, and only operating on a very limited set of routes, and still often completely sold out)