Would not want it to when considering where it could lead, just wondering if an attack on the train would trigger the article.
I think, it would be open for debate and interpretation.
Any deliberate attack/killing of a head of state/member of the government is an act of war.
But they are going to a state in open warfare. Russia could claim , it was a mistake (Ukrainian military target) or collateral damage.
Lets just say, it would be an incredibly risky and IMHO stupid move by Russia aka Putin to target them.
But on the other hand you can argue, that the visiting satesmen are taking a gamble with their lifes and a huge risk of further escalating the conflict.
My guess is, that the majority of non affected European NATO members would not outright declare war or support triggering Article 5. But I think, there would be further sanctions (no energy buys from Russia etc) and more direct support to the Ukraine.
Biggest question is, what would Poland do and which support from the USA would they have in their actions.