Thalys and Eurostar Baggage
 

I will be using Eurostar(London-Paris) and Thalys(Paris-Brussels), and I will be taking a 26" roll-on hard cased bag. Is my luggage going to fit on the racks in the back of the trains? Do I have to pay extra to place my bag at the back of the trains?

It will fit in the racks so long as they aren't already filled. I'd suggest trying to be one of the first ones on -- be prepared to be by the appropriate car if possible. I've found it unusual to see no space available if you are at the beginning of the route.
It really shouldn't be a problem. And no you don't have to pay extra.

>Do I have to pay extra to place my bag at the back of the trains?

There is no "back of the trains" whether you pay or not. If your luggage doesn´t fit either in the overhead racks, between the seats or in the entrance-area rack, you still have the option to place it in the aisle, apologize to every person passing and take it into your lap whenever the drinks cart comes by.

Board your Thalys in Paris as early as possible to snag a spot in the luggage racks at the back of your car. You'll have a little more luck if you're traveling in first (only because there are fewer people per car than in second class). The luggage racks at the back are supposed to be for larger bags (like yours) but you'll probably find a lot of people trying to stuff their 22" rollers into them as well.
If you can go down to a 24", you'll have an easier time of it.
If the luggage rack is full but there are empty rows (not just seats, you don't want to block someone in) or an empty seat next to yours on the Thalys, you can park the bag there, as you'll be getting off at the first stop.
Eurostar is more of a free for all as everybody boards at once. If you can be at the front of the horde (or herd), then again, you'll have first crack at that precious space on the floor level luggage racks.

There will be space, but can you lift your bag? If the racks are already pretty full you will have to lift your bag up to the top of the piles in the racks.

A definite reason to board early. But then you do have to wait til everyone else gets their bags before you can get to yours at the bottom of the pile.

Thanks for all the responses. Yes, I can very easily lift my bag. I will take everyone's advice and arrive early to the station...