If someone (after all that convincing) still can't see the issue then their problem is far deeper than not recognising risk where it should be obvious. What else can be done to educate? It's too late to learn after being struck.
I've yet to find an effective solution for stupid ignorance.
Education.
If someone doesn't understand why something is dangerous, educate them. That doesn't mean calling them stupid or ignorant, it doesn't mean telling them that it's obvious, it means explaining things to them without jargon and without presupposing knowledge of the railways.
I have taken photos (not selfies, I don't do them) from level crossings on many occasions. I've never though put myself in any position where I could not clear the crossing between the audible warning starting and the barrier starting to descend. I'm able bodied and in most cases as I'm at or near the centre of the crossing so it takes me less time to move to a position of safety than it does for e.g. an elderly person who has just begun to cross. I do though at all times when I'm doing this make sure I am aware of my surroundings so that I can react as soon as the warning sounds.
Indeed the only issue I can ever recall at a level crossing was when a cyclist swerved into me when avoiding an impatient motorist (a few weeks ago at Cheshunt).
Is that dangerous? It's certainly not sitting down on the tracks dangerous.
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Tangentially related, the only crossing I have felt was at all unsafe was a public footpath crossing on the Somerset levels (Westbury to Taunton line I think, but it was many years ago) where after entering the railway boundary, the crossing user had (has?) to climb several feet up to the top of the ballast and then descend a similar distance on the far side.
