Why derail trains though, especially in regards to the latter? Given it's one of the most environmental ways for transportation. Besides, groups like that tend to want to take responsibility, the whole point is to get attention to their groups.
Throwing paint at a painting does not help the environment, but it does create radical flank effects.
Blocking the highways does not help agriculture, but it does bring their problems in other people's home / discourse.
Look at photos from these events you refer to, people are proudly holding banners and signs here, not trying to be anonymous and flee from police and intelligence services.
Have you never participated in political activism yourself, and you have no clue why you'd do those things you cite?
But indeed, XR is the least likely subject. They have stopped trains before, but that was a human barricade, and it was a coal train.
Perhaps this is just as well. If Extinction Rebellion really wanted to take down the entire transport network, they could do so by 'accidentally' touring the big stations with a leaky rucksack filled with lead paint fragments or ground up asbestos tiles, to then tip off the press. This would probably reveal an unfortunate truth, that there was plenty of aforementioned toxins already on the network, from brake pads and paint of yesteryear. Nonetheless, some closure for the guys in hazmat suits would be needed.