I am flying from the US to London, than taking a train to Scotland. Not that it really matters, I'm just curious...will I have to go through customs again once I reach Scotland?
No. Nor will you encounter Border Control.
Why the concern about customs? Are you already planning on declaring items for taxation?
Customs is a walk-through at your arriving airport from the transatlantic flight unless you have items to declare.
You don't say when.
This is not a political statement, but a statement of fact, as we know it.
If the UK crashes out in brexit there is a fair chance that Scotland will have an independence vote. If Scotland is independent then you may well have a border issue.
But nobody knows.
If your travel is fairly soon it is likely that the UK will still be one country, so no.
England and Scotland are both part of the U.K., so there is no border. It’s like going from California to Oregon.
Just a point of terminology, but I think you are talking about passport control aka immigration, not customs. The latter term refers to a check of goods you are carrying and declare. That is a non-event. After the passport check, you pick up checked bags if any, and walk out the “nothing to declare” door. That’s it.
Lots of people mistakenly say “clearing customs” when they really mean passport control, but it is best to use the proper terms.
FYI. It’s like going from California to Oregon. Taking the train to Scotland from London is less restrictive than traveleing between Oregon and California.
Anyone bringing in watercraft of any size into Oregon from California (and elsewhere) must stop at a watercraft inspection station. The wagercarft inpsection station for travelers driving north from California into Oregon is the I-5 Ashland Port of Entry.
And all Oregonians (and anyone else) driving into California are stopped at a California Boarder Protection Station at I-5 near Hornbrook.