Are technology cards treated like the colour train cards and discarded once played, or do you keep them for future 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.
From what I'm reading in the rules, there's nothing saying you ever have to discard a technology card. Once you acquire a technology, you keep it, and can use it as many times as you'd like over the course of the game. (Don't think of it like another train card; think of it like unlocking the ability to build certain new routes, or like a permanent upgrade for yourself.)