London to Belgium

You'd be taking the Eurostar from London to Belgium.

It is a little different than taking any other train in Europe. Read the rules and procedure carefully so you understand.

https://www.eurostar.com/rw-en

Be aware that check in time at St. Pancras in London is no less than 75 minutes before departure although I would make it 90. You go through immigration and customs prior to getting on the train in both directions. This way, when you arrive, you just get off the train and go about your day.

I corrected my typo mistake of check in time.

stick to the recommended by the company check in times. Everything happens before you board the train - security, and two sets of border controls, British and French

Don't try to cut the time.... that can be an expensive and time wasting error.

Book early for the best prices.

Last time I went London to Brussels by train, the recommended check-in time was 90 minutes ahead. More recently they’ve recommended 70 minutes. You go through passport control and immigration at London St Pancras hence the amount of time needed.

Other than that, it’s very comfortable and civilised. Second class is very comfortable. I tend to buy food at St Pancras M&S before joining the immigration queues, for price and choice.

There has been some chatter about Eurostar losing their exclusive right to offer this product, but that hasn't happened yet. In fact, Eurostar has taken over former Thalys services.

The train goes only to one of the 3 Brussels downtown stations, Midi/Zuid. But your ticket allows you one local train (not city subway ... ) trip to Centraal or Noord. Those local trains are very frequent at normal times, and unreserved.

There are a few private operators planning to offer competing services on the line but they won't start until week into next year at the earliest. And there may not be services to Brussels anyway.

“The train goes only to one of the 3 Brussels downtown stations, Midi/Zuid. But your ticket allows you one local train (not city subway ... ) trip to Centraal or Noord.”

This indeed used to be the case, but not any more. From December 15, 2024 onwards, Eurostar tickets to Brussels stopped being valid in the entire Brussels train zone. You therefore need to buy another ticket if you want to take the train from Brussels Midi/South/Zuid to Brussels central or any other train station.
Source; https://help.eurostar.com/faq/uk-en/question/I-ve-bought-a-ticket-to-Brussels-Midi-Zuid-can-I-use-it-to-travel-to-another-station-in-the-city

What I did was Eurostar to Lille, and rented a car from there. If you only want to visit cities, then yes, trains are fantastic in Belgium. But if you want to visit battlefields or countryside, a car is very useful. I had the most delightful drive from Bruges to Zeebrugge and up the coast along the beaches to Breskens in the Netherlands.

Dutch_traveler, thank you very much for the important correction.

I just realized that the OP said "Belgium", not "Brussels". So he/she might consider the "Any Belgian Station" add-on to Eurostar, if that is still available, if he's immdiately going beyond Brussels. It saves the trouble of buying another ticket, if not that much money for a nearby destination. Note that there tend to be around five trains an hour on the corridor between Bruges/Ostend and Brussels, in the central part of the day. All unreserved. Some posters here have reported success using American credit cards in Belgian train ticket machines.

I'm sure there is a taxi stand at Brussels Midi, as well as a subway station. I assumed he might be going to Centraal, for either Grand Place, or the mass of museums and other sites nearby. I have not been to Brussels in 10 years, but I should note that some posters consider the area around (.... outside) Brussels Midi to be less safe than, say, around Centraal Station. But as a New Yorker who knows how safe NYC is among American cities, I am reluctant to repeat hearsay as if it were a fact!