The train will add about 5 hours to your journey and involve unnecessarily passing through UK immigration. And it's worse on the way back.

If you are considering a train, AMS is a little over two hours from DUS and BRU and about three and half from FRA. Don't know if you will get better prices but it does open up some more options. And no additional passport checks...

BA fly from LGW to AMS as do Easy Jet.

The train from St Pancras to Amsterdam is quite easy. There are two direct trains a day (one in the morning and another in the evening) but more options with a change in Brussels. Check the Eurostar Website for options.

Given that the Thameslink train from Gatwick stops at St Pancras it makes for a smooth transfer.

The train on the return does require a change at Brussels but to be honest it's not really a hardship and in some ways is much easier than using an airport.

I'm surprised you are recommending this. Everything you said is correct but it just doesn't seem very sensible.

If they had time for a proper stop over to visit London then the Eurostar might be a good idea but they say they are going for work.

The TATL is 10 hours. If the OP wants to pay a bit more to be a bit less uncomfortable in premium economy, and the cost comes down by adding a stopover in another Schengen airport - which would add perhaps 3 hours to the overall travel time - I think that would be worth it. There will be no additional security screening and passport control will take place at the transit airport instead of AMS, so no different to a direct flight.

Alternatively flying into DUS or BRU or even CDG followed by a train would be worthwhile too if it was cheaper overall, you arrive directly into Amsterdam at the cost of a small risk of being delayed long enough that you need to figure out a new walk-up train ticket, but similar travel time to a connecting flight.

Connecting at LGW on one ticket would also be fine but requires going through security again at LGW. On separate tickets, unless you book it through the GatwickConnects+ service which probably removes all cost savings, you have the hassle of UK immigration twice, security twice and collecting and rechecking bags twice, and you need to present yourself at the easyJet visa check desk even if HBO, so need a suitable buffer for this. For the incremental increase in comfort of Norwegian Premium I don't think it is worth it.

The Eurostar has all these negatives and looking at the timetable it seems even worse. The DY flight from LAX arrives at LGW 1400 today, and a direct flight to AMS would take a similar time. If you want to add the Eurostar, realistically you should book the 1731 train arriving 2212, as after immigration and collecting luggage it might be 1530 by the time you get to Gatwick station, then you need to work out your ticket and hope Thameslink is actually running, so get to St Pancras just before the recommended cutoff of 1700 (and the Eurostar area is not exactly next to the Thameslink A and B platforms). On the return you need to catch the 0748 Eurostar to make the 1400 DY departure to LAX.

If flying in a flat bed or a really cheap flight or you specifically want to experience the Eurostar then fine, otherwise it seems silly to subject yourself to all the waiting and running about just for a slighly wider seat.

The OP asked about doing the train so I was just informing them that it is a possible option for them to consider. Is it the easiest option then no it's not but it surely is an option. Just as an option would be WOW LAX-KEF-AMS or the direct DL flight to AMS. OP could also fly BA or VS with a change at LHR. or SAS via CPH

I'll leave it up to them to decide if it is sensible or not for them rather than just rule it out of hand which you appeared to have done. They said the trip is for work but that does not preclude a leisure element.

The distance between the Thameslink and Eurostar platforms is actually quite close - closer than some gate to gate treks at airports - I've done it a couple of times (though not after a 10 hour TATL) and it's all step free with lifts and escalators from the Thameslink Platform to the concourse level. Buying a train ticket is hardly difficult either - there is plenty of advice on here on how to do it and the OP can always ask for more info.

Hi,

You will need to clear security at London prior to London-AMS but not customs or immigration ( unless you were changing airports) as Customs immigration would be done at AMS.

This is assuming you are on one airline and can be checked through to AMS transiting through the same airport.

However note that some airlines ( ie BA) will not check baggage through on separate tickets even on the same airline ( BA)

Regards

TBS

OP is dreaming if he thinks he will spend only US$2K on Business Class RT tickets!

I doubt the OP will find real business class to Europe from the west coast for less than $2K
They may get a "business class light" such as Norwegian/WOW or premium economy with BA or AF/KL
I would suggest looking for flights from SJC or LAX;
LAX consistently has much cheaper flights West Coast US - Europe than SFO

A couple of options come to mind.

1) If the dates line up, you could fly Icelandair SFO-KEF-AMS in Saga Class for $2,400 a person. It's not lie flat business, but it's a lot better than 9 or 10 across coach in the back of a widebody.

2) Somewhat surprisingly, Google Flights offered up Air Canada / Jet Airways SFO-YYZ-AMS for 2,400 a person for some dates in October. Jet Airways is lie-flat business although I have no idea what it's like otherwise. There is a nasty 17 hour overnight layover on the return, though.

I hate 10+ hour flights in coach, but I'm still not sure how far out of the way I'd go vs a non-stop coach flight. a connection sure. overnight layovers, not so much. But that's a personal call.