In reply to HardenClimber:
I went via Eurostar to the Alps this summer and used eurodespatch for axes, crampons and climbing kit - had read various stories of security sometimes being ok, and other times not (and had similar feeling to you of it getting a bit tighter in the last year or so).
It worked, but certainly not as straight forward as checked luggage on a plane.
Outbound: We were getting eurostar to Paris, then changing onto TGV trains, so needed our bag to be on the same train we were on otherwise we'd miss connection. I spoke to eurodespatch on the phone, and went to the desk at St Pancras and was told for bags I didn't need to book, and as long as I dropped the bag an hour before the train then it would definitely travel on the same train as me. However when I went to drop my bag off at eurodespatch on the day, I was told it was only a 24hr service and they couldn't guarantee my bag would be on my train. After some discussion around merits of eurodespatch vs trying my luck with security it managed to get a spot on my train.
Picking it up at Gare du Nord was very straight forward bag was waiting for me when I got to the desk (this is a long walk from where the train arrives - maybe 10/15min each way)
Return: Dropping bag off at Gare du Nord super easy, was put on the train 60mins after I dropped it off. Had heard you need to wait for a customs official to check the bag in, but didn't seem to be the case - just gave it to the person at the desk and paid the €40 then it was done.