The capital of the EU to the capital of France in an adjacent country is pretty much the definition of good train travel in Europe. Also anything TGV in France. As sibling noted, Eurostar from London is also great. Various other routes are pretty good.

Prices are pretty high by and large especially if you haven't booked well in advance.

As someone else mentioned, very fragmented in Europe generally. I actually try to take trains in Europe but not always a great option especially when traversing multiple countries.