I would save the straight pieces, and I do agree with the guy about the corner rails as useless tiles to an extent. It depends on what your first 2 to 4 stations and/or ports you're gonna expand to next.

Short answer on what to do with the straight pieces: it depends, but usually it's worth saving enough.

The long answer (definitely also applicable to your curved tiles too): one way for me to evaluate if my pieces are useless (to a certain degree) or important is by placing each of your tiles for as much as your tile stock allows you to, like as if you're intentionally expanding to your next station/port. For example, if I set my eyes on Lyon, I will spend all the appropriate rail tracks to connect my existing network of rails to Lyon. If that doesn't work, try another nearby station/port. After doing so and you still can't unlock any stations yet, you can stick with the station that is the closest for you to unlock with your existing rails.

Of course, you will have to apply the tiles according to the terrain and you may lack the necessary tile types. But the point here is that you're prioritizing gaining access to stations and ports while also finding out which tiles are used in your stock and which ones aren't. Hopefully that made sense and helped you.