The challenge for David Cameron in setting out British terms for the renegotiation of its relationship with Europe, was always that he should not set the bar too high. That he would demand enough to show a seriousness of purpose , enough that was actually negotiable to be able to show results. But not demand the impossible, the unachievable, creating a a stick with which Ukip and his less loyal backbenchers would beat him, and failure to achieve which would force him on to the "leave Europe" side of a referendum campaign.
That's the challenge if, that is, one takes at face value the British Prime Minister's professed desire to keep the UK part of the EU. It's difficult to do so, however, following Cameron's indications in the last few days that he not only wants to reform welfare entitlements of EU immigrants to Britain, on which he may find common cause with Chancellor Merkel, but is setting his Don Quixote-like lance at the whole principle of the free movement of labour. And painting a red line that Ukip et al will never allow him to repudiate.
The 27 other member-states, he knows full well, can never – must never – agree to undermine one of the fundamental pillars of the European project, one that until recently the Tories themselves acknowledged was an essential corollary of the free movement of capital and goods and the single market. That single market which has always been, irony of ironies, the sole acceptable Tory rationale for the whole European project.
The problem now for those like Merkel, or indeed Enda Kenny, who might be willing to go the extra mile to keep the UK in, promising to negotiate and urging moderation in their fellow member-states' responses to London demands, is that Cameron is making their position as untenable as he has made his own. It looks increasingly as if the whole "Brexit" train is simply gathering steam unstoppably like Denzel Washington's train as it lurches towards that fateful bend. And calamity.