I know when you book a flight and skip the first leg it would cancel all remaining flights. But say for example a route AMS-BRU-BHX, where the AMS-BRU sector is on the train, would I be able to skip that sector and travel direct to BRU to catch my flight, or would it still void the rest of the ticket as it would on a flight sector? If you did this, how would they know (do they check you in at the station as you would on a flight?)

Reason I'm asking is that I will be roughly halfway between AMS and BRU, and a direct ticket from AMS or BRU is 3 times the price of the itinerary with the surface sector.

Appreciate any help!