Purchase eurail ticket from London to Brussels same day likely

Eurail is not a train company. It is a travel agency that rips you off.

Please read this article for instructions on how to buy tickets for a London to Brussels journey - https://www.seat61.com/Belgium.htm

Do you mean "Eurostar" ticket?
https://www.eurostar.com/us-en

The train company that runs from London to Brussels is Eurostar at www.eurostar.com

Are you asking if you can use your Eurail pass to purchase a ticket at the last minute? The website suggests making a reservation which makes me think they've got a specific number of slots for pass-holders. The trains can sell out for paying passengers as well so if you've got a specific time you want to travel you should reserve after making sure the interrail pass is right for you. There is a fee for it looks like 30E for the seat reservation.

https://help.eurostar.com/faq/us-en/question/Can-I-use-my-interrailing-pass-on-Eurostar

**Editing to add: Oh, I see why you want to book at the last minute since you are flying in to Heathrow and trying to connect to the Eurostar the same day.

You will have only 3 likely options of trains on April 16th, 15:04, 18:04 and 19:34. As seats sell between now and the 16th the price increases and a train can sell out. If you have a Eurail pass you should buy a seat reservation for one of these 3 time slots, probably the middle one—the first one cuts it too close with your arrival, immigration, transportation to St Pancras and one hour check in time for the Eurostar to clear immigration at the train station. If the seat reservation allotments for Eurail pass holders sell out, you will need to buy a ticket at full price. Also if you plug in a date much closer to today’s date you will see that some of the trains and classes of tickets have already sold out.

You’ve put yourself in a very risky situation with this plan you’ve been asking about Red Flags in several posts. Have you considered:

*Buying a ticket to fly from LHR to BRU in the late afternoon of your arrival knowing you might not be able to take the flight if your US to London flight is severely delayed? There are 3-4 flights an afternoon from LHR to BRU for about $78 that will get you there in 1 hour.

*Spending the night near St Pancras and booking the Eurostar on April 17 at a time you know you can make?

What ever your group does you should soon and with the least amount of risk, as you are 2 months away from your trip and prices go up. It’s all part of of the decisions we make and learn from when booking tickets to a city when we need to be somewhere else. I’ve done it several times but I always factor in the cost of a hotel, ticket that’s thrown away or new ticket that to needs to be bought on the spot. Personally I think that a very inflexible plan is more prone to problems.