I've mapped out what feels to me like a huge area of the fulgora subway and haven't found a trace of either of the scrap types the research seems to be expecting me to mine. Am I missing some mechanic or is it just really really rare or something? I spent like an hour running through with quite a few speed upgrades and I haven't seen anything but long hallways without any resources and train platform looking rooms with nothing useable in it.

Have you shot the doors? This spawns new rooms, and also a wave of enemies.
Have you tried tunnelling down to the subway while standing next to a Fulgoran Vault on the surface? The new scrap types spawn underneath the vaults