According to a news report, by 20th December. Apparently it was already planned following the attempted mass shooting on a Thalys in August, although the Paris attacks can't have helped.
Spain has been doing this for AVE highspeed trains since the Atocha bombings and I assume this will be a similar system. Eurostar also does it. I would also expect more rail systems to implement it, such as TGV. In Spain, the checks are much less intrusive than at airports - mostly consisting of an xray scan of luggage and walking through a metal detector.
Personally, I welcome it.
The first ones that will be installed, according to press I'm seeing, are in Lille and Paris Gare du Nord for Thalys trains going to Belgium. The French transport minister said the French will work in cooperation with their other partners in Thalys (Belgium and the Netherlands) on installing additional security as soon as possible in Brussels and Amsterdam as well (and, one report said, with the Germans to do the same in Cologne).
According to other press reporting I saw, folks traveling from Brussels, Antwerp, and Paris Gare du Nord on Thalys etc have already been advised the last few days to arrive at the train side at least 30 minutes before departure due to "reinforced security measures" before boarding.
And here I was saying on some other thread the other day that this wouldn't happen in the near time! Shows me what I know!
Fine with me too.
But digging around a bit, I see a lot of news about new gates but nothing that confirms airport-like checks. France24 states that:
"...the company would not comment on exactly what form the security checks would take, saying it was “a matter entirely for the authorities in the countries which want to put in the gates, it is their responsibility”.
http://www.france24.com/en/20151124-france-thalys-security-checks-european-trains-terrorism
Similar from Reuters:
"She did not specify what kind of security gates would be installed."
I'm seeing other mentions of measures which have been proposed but not approved yet. Kinda sketchy?
In addition to having your luggage scanned, you must be prepared to show valid ID before boarding. For non-Europeans, that's your passport.