Just wanted to find out if I should buy the train tickets in advance for travel from Brussels to Amsterdam in Sep or can I do it at the train station. Thanks in advance.

That depends on whether you want to use Thalys, in which case you need to buy your ticket ahead of time to get it cheaper, or are happy with the slower Intercity train, in which case you don't need to but ahead of time.

If you look at the Belgian Railways website you can see the trains available, the time they take and their prices.

www.thalys.com to book your own Thalys tickets - at a sweet discount if you book early enough but much cheaper than full fare- could be cheaper than slower IC trains but only if you book in stone -no changes no refunds from a specific train at a specific time.

Otherwise IC trains have a flat fare - much cheaper than Thalys unless you get a discount on Thalys. But no reservations even possible - hop on any of the about hourly IC trains that do take about an hour longer. For lots of info on trains check www.seat61.com; www.budgeteuropetravel.com and www.ricksteves.com.

Option 1, Thalys high-speed, in 1h55:

From €25 in advance, rising to around €80 on the day. Reservation required, can therefore (in theory at least) sell out.

Option 2, hourly Intercity (IC) trains, in 3h20:

From €19 in advance, regular fixed price fare around €50 which you can buy on the day, unlimited availability, no reservations necessary or possible, cannot sell out.

Both can be booked at Belgian Railways www.b-europe.com

IC trains Brussel-Amsterdam are often very crowded at times in 2nd class - discounted tickets also come in 1st class usually (not sure here) and could well be worth it -rare that 1st class would be SRO as I've seen 2nd class at times.

Thalys where reservations are required does not have this problem and 2nd class is very comfy (but as always 1st class more so and in this case includes a full meal IME with wine or beverages of your choice.