Ticket to Ride United Kingdom: Are technology cards kept after use?
See Purchasing Technology cards as a way to upgrade what you can do. Most of the cards are bought and kept in front of you helping your development (allowing you to claim longer routes or opening new areas for you to build).

In fact there are two technology cards that you don't keep but it's explicitely written in the rules: the "Right of Way" technology card and the "Thermocompressor" one (in the Advanced technology variant). You purchase those cards, apply them and return them to the pool for other players (or you for that matter) to purchase them again later.