I guess these waste train problems don't make it to the news on the other side of the pond, so let me assure you there is 0% chance the train would ever drive over the protesters. So, it's about as safe as chaining yourself to the locomotive door so that the train driver can't board the train in the first place to drive it anywhere. It's nothing but a nuisance to the train drivers and the society trying to deal with nuclear waste. It's not a safety issue per se, aside from standing a train full of nuclear waste in the middle of nowhere. Like Ryll said earlier, they know beforehand of the protesters, sometimes even so much beforehand they can try to use a different route (if available). So, in short, the only harm the protesters might face are, surprise surprise, the angry police officers.

To borrow the famous quote, I personally know a train driver and it's offending to even think they would be so worthless people they would drive over a mass of humans. Even driving over a lone thoughtless suicider can be an emotional burden.