It _really_ depends on the distances and sizes of the cities you're going to. Paris-London train is about 3 hours, which is likely less than you'll spend on getting to+from an airport. Paris-Berlin on the other hand is miserable, unless you love trains.
Or unless you take a night train. Get in in the evening, sleep most of the way and wake up at your destination. That can actually save you time compared to a flight, or be a lot more comfortable than, e.g., getting up at 4 AM to take a flight to a meeting in the morning somewhere.
I was speaking more generally, sorry; there is currently no daily Paris-Berlin night train (and I guess the weekly Russian train is suspended). The most reasonable connection seems to be: Paris 19.04 (ICE 9557) 22.17 Mannheim 23.47 (Nightjet 470) 07.38 Berlin, with some time to drink a nightcap in Mannheim, but I can see how that won't work for everyone. You can also leave Paris at 17.55 and change more comfortably in Karlsruhe with maybe a late dinner. In the reverse direction you'll have an unpleasant change in the early morning though.
In daytime, there are a few options with only one change, but you'll sit in a train for 8 hours or so, which may not be your thing.
This is exactly my point, trains can't compete with point-to-point travel at 500mph. So injecting a bunch of padding on either side of flights is the best way of making train travel more appealing.
Those points are usually fairly far outside of their respective cities though, to be fair. Planes are always going to be preferable for long journeys, London-Istanbul is a trip only big train fans would plan to take by rail. But I can take some really nice direct train journeys where I am - Budapest, Berlin, Munich, Vienna, Belgrade and more. And since I end up in the city centre it's usually a quick tram or subway to where I'm staying, or hopping another train onwards to a smaller town. And travelling on the train with friends and a beer is really pleasant :-)
