Seat reservations are not possible on trains in Belgium and the Netherlands, whether you have a ticket or a pass. Hop on and sit wherever you want. (TGVs, Eurostar Blue and Red have mandatory reservations while on ICE trains they're optional). This system works well in multiple countries, don't worry. Trains are long and frequent so you'll always find a seat.

As a tip : people are generally lazy and board in the middle.

If you'd like to avoid paying 22€ for such a short ride here's the route I'd suggest (fully reservation-free) :

- IC Amsterdam Centraal - Antwerpen-Centraal 11:28 - 13:16

- IC Antwerp-Centraal - Kortrijk 13:36 - 15:01

- IC Kortrijk - Lille-Flandres 15:13 - 15:51

The change in Kortrijk is usually not necessary, the same train goes all the way to Lille.

Do not hesitate for questions.

Thank you! 

I'm actually trying to get to London that day but all the trains from Amsterdam are already booked; my fault for waiting till now to try and book seats. So now I'm looking for other alternatives to get me there as my hotels are already booked so I have only certain days to travel on. Looks like I can do Amsterdam to Lille and then Lille to London on October 1st.

Use this to check and book Eurostar https://www.b-europe.com/EN/Booking/Pass#TravelWish

On 1st Oct the last train with  2nd class availability is the 15.30 from Lille, there is also 1st class available on the 18.35, only if you have a 1st class pass.

You should plan to arrive in Lille an hour before this as all checks are done at Lille Europe before boarding.

Lille Flanders is a simple walk to Lille Europe, less than 10 minutes.

The reservation free connections @thibcabe gave run hourly but are slower on a Sunday so to get the 15.30 you would need to depart Amsterdam at 08.28 changing in Antwerp Central (also called Anvers Central) and Kortrijk (also called Courtrai) to Lille Flanders for 13.51

Ignore the note on the rail planner about a supplement being required on this, it is incorrect but has not been removed.

You should book your Eurostar connection urgently as they have quotas for passholders and do sell out.

Okay now that we have all info I can give some other options :

- IC via Kortrijk, Eurostar from Lille 30€

- TGV via Brussels, Eurostar from Lille 50€

- Eurostar Rotterdam - London St. Pancras 19:28 - 21:57 35€

- daytime or overnight Stena Line ferry Hoek van Holland - Harwich -> really convenient (used it 10 days ago), 30% discount with Eurail = tickets at about 30€

For more info about the ferry (fun experience) : https://www.seat61.com/trains-and-routes/london-to-amsterdam-by-ferry.htm

As you can see the Eurostar is a real bottleneck as is crossing the Channel/North Sea... :(