Thalys train from Paris to Brussels

First, forget about Rail Europe. They do not list all trains, and they mark up tickets they do sell - sometimes substantially. Unless you have a very specific reason to use them, avoid them.

You do not need to buy tickets in advance, provided 1) that you won't mind if some trains are sold out and 2) you don't mind paying top price (€99 one way!). Since you want a same day return trip, you will probably want an early train to Brussels and a late train to Paris - exactly the ones likely to sell out. You will also be able to commit to trains (the cheapest tickets aren't refundable or exchangeable; if you don't take the exact train you booked, your ticket is now scrap paper).

To see fares and options, go directly to the Thalys website: https://www.thalys.com/de/en/. Book tickets directly here; you should get an e-ticket e-mailed to you. Fares can be as low as €29 each way if you book far enough ahead. Today, you can book up to April 18 - and already, some of the cheapest tickets are sold out! So, don't delay; prices on these trains never go down, only up.

Bill, No, and neither plan is good. Do not buy from RailEurope as they are a reseller with a reputation for markups, and do not buy on the day.
The best place to buy is direct on the Thalys website: https://www.thalys.com/be/en/
You will get a print-at-home ticket e-mailed to you, which is what you need to show on the train.

Tickets are a lot cheaper if bought in advance. Only people with urgent travel plans (and are prepared to pay) buy on the day. When you buy a ticket it is for a specify train (It has date, time, coach and seat number on the ticket), you cannot then change which train you are taking.

Ditto to the above: buy your tickets in advance from the Thalys website. We've taken this train from Amsterdam and Paris (it stops in Brussels) and you don't need to purchase from third party (RailEurope).

Is Raileurope still in business ? I know it is or was owned by the French Rail or SNCF geared toward the english market before the advent of google translate or sites that allow bookings in multiple languages.

Yeh, just click on the rail links on this website. It has its place, but definitely should not be anyone's go-to site for information or ticket purchases.

Jazz+Travels, I always thought RailEurope was based in the US and geared towards the North American market (even though it is owned by SNCF). It has prices in US$, times in am/pm, back-to-front US dates, defaults to English and has the look and feel of a US website, all designed to give a comforting feeling to North Americans (and maybe other non-Europeans) who would perhaps feel worried about buying on a "foreign" (I.e. French/German/Italian/UK etc.) website. The English would not use a site priced in US$.

Bill, before buying your first rail ticket, read this site, it is the best primer on how trains work on the net: http://www.seat61.com/Europe-train-travel.htm

Bill, Thalys sells tickets exactly like an airline: Non-chabgeable, non-refundable prices like (these, made up) $39 three months in advance at non-peak hours, $59 more desirable hours, $149 day of departure. You have to decide how much it's worth to be footloose and fancy free. Don't use RailEurope!

A question, please...does the Thalys site offer alerts for upcoming travel? I can't see it. Thanks

Denny, what sort of alerts are you looking for?
Trains, unlike planes, generally do not change their times at the last minute, so there is no issue of having to alert you to that, and they are frequent. Tickets are first offered ~90 days beforehand, and only ever go up, not down (technically the cheapest unchangeable, unrefundable fares sell out first).
What do you want to be alerted about?

Two practical points.
You'll probably have a security check at the Gare du Nord in Paris. Be there in time (half an hour before departure).
In Brussels you'll want Brussel Centraal as an entry point to the city. This station is not served by the Thalys, which only stops at Brussel Zuid/Bruxelles Midi. Change there for the next train to Centraal. They leave every few minutes. The price is included in your Thalys ticket.

Thanks to all who so promptly and insightfully replied. Appreciate the very useful advice.

Bill

Rail Europe has the Paris-Brussels Thalys ticket from $35, through their direct connection with Thalys, so there's no significant price difference.

FYI, Raileurope.com is based in the US to serve the US market, with main offices in the New York and Chicago areas. For delivery to Canadian addresses, they have a .ca extension and ship from the Canadian side of the border. The British version .co.uk I think did change or merge recently; it never offered as wide a range of products. Raileurope-world is for delivery on other continents.

Chris, I was thinking of alerts that advise when tickets for a particular travel date become available for sale. On Dec 8 I signed up for alerts with virgintrainseastcoast for May 2017 travel in the UK, and the next day was alerted that tickets for 22 May were available for purchase, much sooner than I had thought. In 2015, I am almost positive we bought highly discounted SNCF tickets from Paris to Colmar months before other needed tickets were available. I know they're all different operations, just want to get it right. We will travel from Amsterdam to Paris 22 May 2017. If Thalys is always 90 days in advance only, that's the date I'll mark and cool my heels. Thanks.

Denny: Since on December 18 I was able to see tickets for April 18, it looks like Thalys tickets become available 120 days before travel. If you can't find a way to set an alert on their website, just set one on your phone's calendar.

There used to be a raileurope.co.uk for British web bookers, which was simply an English-language and pound-priced front end to the normal SNCF site without the dodgy games that the .com site plays. The address now simply redirects to an English-language booking page on the main SNCF site.

Harold, thank you. I re-read your post. Part of the problem is I can't count. I'm looking at train tickets for a few different countries on two different trips a few months apart. I'll get it straight yet, I hope.