Can someone further explain this from the Eurostar website: "...your Eurostar ticket is valid for your onward journey to any other Belgian station." How does this work?

Would I have to make train reservations online/at the station?

Here is a quote from www.seat61.com that should answer your questions: "All Eurostar tickets to Brussels are automatically valid to any station in Belgium. You can use any reasonable connecting train service from Brussels to your final destination in Belgium as long as you complete the journey within 24 hours of the Eurostar arriving in Brussels--just use the online timetable at http://bahn.hafas.de to check train times. Allow at least 25 minutes in Brussels to make a connection on the outward journey, and 45 minutes (preferably a bit more) on the return to allow for the necessary 30-minute Eurostar check-in. Similarly, on the return journey you can leave any time within 24 hours of the departure of your Eurostar from Brussels back to London." So no reservation is necessary. Just get on any train that is going where you wish to go.

Apart from on the high-speed international trains, reservations aren't really a feature of the Belgian train system. It's mostly like a big city suburban railway network, only rather cleaner, more reliable and with better-behaved passengers than some I could mention.

Just turn up and get on.

You cannot reserve seats on domestic Belgian trains. On most lines, trains run every hour or every half hour.

Remember, also, that the class of your Eurostar ticket determines the class you ride on Belgian trains. So if you have a first class ticket, then you can ride first class on Belgian rail (Thalys excepted, of course, but the Thalys trains aren't part of the ABS arrangement).

I believe it's 48 hours free ABS ride in conjunction with going to or from Brussels for a Eurostar trip.

Hi there - we just did a weekend trip to Bruges using our Eurostar tickets. Like the others, just hop on the train and go. When the ticket inspector comes, show your Eurostar ticket. It's that simple. My one tip would be to travel light if you can. I saw a lot of people struggling with heavy suitcases and the trains were sometimes full with people standing..

To clarify my reference upstream, travel to another station in Belgium must commence within 24 hours of arrival in Brussels. (This information comes from both seat61.com and raileurope.com.)

I have also seen 48 hours, but maybe i'm mixing that up with Thalys ABS tickets - i'll have to check it out.

I don't think Thalys offers ABS tickets.

Sorry... what does ABS stand for? Would the Eurostar ticket be valid for use on a direct train to Amsterdam, because it is part of the Benelux countries?

ABS = any Belgian station. No, you would have to buy a separate ticket to Amsterdam.

ABS = All Belgian Stations

Btike: Yes Thalys tickets do come with an ABS option - that is regularly priced Thalys tickets - Raileurope is currently selling half-price Thalys tickets through August and cautioned that the usual ABS Thalys fare would not be available on these tickets but only on regularly priced tickets.

Clients holding Thalys tickets to ... Any Belgian Station (ABS) Fares. This special price enables client to ...
www.eurorailways.com/brochure/premier/thalys.pdf
This explains that ABS fares do exist with Thalys tickets - but not all as some discounted tickets do not qualify.

On the Thalys website, it's marketed as something you have to pay a supplement for. It's a given with Eurostar tickets to Brussels, regardless of the price you paid for the ticket.
http://www.thalys.com/be/en/any-belgian-station
Even when available with the Thalys, price shopping is important. Belgian train fares, particularly on week-ends and holidays are quite inexpensive in second class. So buying the cheapest Thalys ticket then paying extra for a b-rail ticket may sometimes be cheaper than buying a more expensive Thalys ticket with the ABS supplement.

There is no supplement, however, for the ABS fare when bought in the U.S. thru RailEurope, except for promotional tickets like their current 50% full fare sale through the end of August.
I thought in online sales from Europe there could be a charge and your comments about it are valuable - something you wouldn't blindly buy i guess - but if it's free like thru REurope then you take it.

I looked at your link and those people have some errors in it: for example, it lists car 5 as having 56 smoking seats in second class when the Thalys web site clearly and plainly says ALL Thalys trains are nonsmoking (this could be a real problem for a smoker who buys a ticket and thinks he has the right to light up in that car). And that pdf spells Brugge/Bruges as BRUGGES. That's so unprofessional! (but now I know why so many Fodorites mistakenly spell it that way)
If it were me, I'd compare any "specials" that these people are offering with the cheapest fare available online from Thalys itself and the cost of an 2nd class ticket within Belgium.
Also, why do these people traveling in first class on certain routes get no meal? There's nothing about that on the Thalys web site....is this a restriction only for people who buys through eurorailways?

Well i certainly don't endorse nor know anything about that link - it just came up when i Googled Thalys and ABS fare - my only point was trying to document that Thalys tickets do come with ABS fares and that's all - i would agree go to www.thalys.com or www.sncf.com to compare prices.
RailEurope's Thalys info sheet says that meals are not included in first class on Amsterdam-Marne-la-Vallee (EuroDisney) trains - that may be what they're talking about - i think all first class tickets get meals on other runs but not sure.
The sheet also says all Thalys trains are non-smoking and have been so for several years so the site i referenced seems seriously outdated.

Yes, the link may be outdated. But the current Thalys site does indicate meals are served to 1st class pax on trains from Brussels to Marne la Vallee. So I wonder if that's a eurorailways thing (and if it is, whether the Thalys staff actually enforce it). I've never ridden first class in Thalys (about 95% of my Thalys trips) when they didn't serve the meal to everyone in the compartment. And that includes trips on the MlV route.