Train from London to Paris how early do I arrive

For the trip from London to Paris, I suggest arriving 1 hour before departure time but you can check the Eurostar site https://www.eurostar.com/us-en/travel-info/your-trip/check-in. There is passport control for the trip from London to Paris and that is done in London. For the trip from Paris to Frankfurt, arrive 30 to 45 minutes before departure time. Usually, the track is posted about 20 minutes before departure time but I give myself some extra minutes. There is no passport control or customs for the trips within the Schengen zone, which includes France, Germany, and Belgium. Having never taken the train from Frankfurt, I do not know how far ahead of time you need to arrive, but I'm sure you will get other responses.

Very helpful!

Watch for holidays and perhaps on the weekend I would be earlier at London. We hit the Eurostasr during a holiday and an hour and half was barely adequate for check-in and getting through immigration. For regular train service within the Schengen zone 30 minutes is adequate. Know your train number. Often the train board will list a number of stops for that train and it is possible that your destination is not listed so don't panic. The train number is key.

Very good advice

You will also go through UK immigration in Brussels on your return journey. When you arrive in London you just walk off the train.

Look at the platform for a display titled »Train composition« or something similar. It well tell you at which part of the platform (indicated with capital A, B, etc.) your carriage is likely to be found.

The Eurostar (London-Paris and Brussels-London) is an exception to the normal rule about not needing to arrive early at stations. Because there a security check and both sets of passport control (UK exit AND French entry) before you get on the train, you must be through the ticket barriers 30-45 minutes before departure time. 1 hour is recommended for newbies.
There is no customs. There may be after Brexit, but I doubt that will add to the time.

Any other international train, just treat like a normal train. There is no passport control within the Schengen Area. Arriving at the station 30 minutes before departure will give you enough time to buy a sandwich/coffee. 10-20 minutes before if you know the station. The only way you will know you have crossed a border is because the language of the on-train announcements will change.
Outside the Schengen Area, the passport check will be done on the train, not before you get on.

This is a major major help and so appreciated

One Sunday in the summer I arrived at St-Pancrs an hour early and joined a lineup so long it was nearly out the front door. Some of the crowd were there for the train scheduled to depart after mine. But the lines moved steadily and the attendants herding the crowd clearly had plenty of experience. My train was 10 minutes late departing to be sure everyone was on board. So go early, with a heap of patience.

In general, European train stations post the location of the trains -- that is, the platform of departure -- about 20 minutes early. There are big boards showing platforms and times. The train numbers are shown, and the final destination of each train, so it helps to know that city even if you are getting off at an intermediate stop. Some stations, such as Gare de Lyon in Paris, now have turnstile entrances to the platforms for ticket scrutiny. Others still allow passengers to walk straight onto the platform. For long-distance trains there is often a diagram showing the location of the various cars by the platform.

For regional trains, with a ticket bought at the station, it may be necessary to validate the ticket by stamping it in a machine at the platform entrance. Trains with advance seat reservations don't.

"Customs" handles what you are importing and generally involves walking through a green door marked "nothing to declare". Immigration and border security procedures will take time when you land in the UK and at St-Pancras for France.

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One hour is the absolute minimum for Eurostar. I would be there at least 90 minutes before to be safe.

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