I have to admit that I'm struggling to see the direct problem. From what we can tell from the photo, the couple are standing in a location (and, presumably, at a time) when it would be perfectly legitimate to be walking across the crossing. Presumably, if the crossing barriers were about to go down, there would be an audible warning, which would alert anyone walking across the crossing that they do need to get off it as quickly as they can. I see no indication in the photo that the couple are doing anything that would prevent them from immediately doing exactly that if that warning was heard. On that basis, they don't seem to be an a situation that's any more dangerous than if they were walking across the crossing. Of course you can speculate that maybe there's something else going on that is dangerous - but you can't deduce that from the photo.
I appreciate that there is a slippery slope argument where some people might continually try to push the boundaries of what is acceptable - and the consequences of pushing that boundary can be absolutely awful. You certainly don't want to be encouraging people to be stopping or taking photos on level crossings - because there is a very fine line between that and doing something that clearly is dangerous. But at the same time, I'd have thought that if NR wants to reinforce the message of safety on level crossings, or start threatening people with prosecution, then they should be using examples where people are doing something that is definitely dangerous. And this isn't an example of that. This looks like an example where people are straying quite close to the line of 'dangerous' but haven't actually - from what we can tell - crossed that line.
Having said that, I can see that NR are probably in an awkward situation. Once their attention has been publically drawn to the photo, it's pretty much impossible for them not to respond at all - as that will leave them open to accusations of encouraging undesirable behaviour. But it's probably almost impossible to frame a public response on Twitter that adequately captures the nuances of explaining that it's only a very small step from what that photo shows to actually putting yourself in serious danger - and isn't open to misunderstanding. Perhaps in that light, NR's response really was the least bad option they had?