Well, Highpool and Ag Center (as well as the "Infected Sites") are very "mission-based", you receive a distress call from both and have to choose which one you will respond to, that's why they feel a bit railroad-y, I say "feel" because nothing about the game is actually railroading you, you can just ignore the distress calls from both locations and let them die, the game supports this and you can still continue the main quest, though the Rangers aren't too happy to say the least that you let both die. In a way Highpool/Ag Center are "starting dungeons" in RPG parlance.
After that, well you can go anywhere on the world map, you do receive some distress calls which you can respond to or not, but most locations are not in immediate need of attention. Rail Nomads for example doesn't really have a traditional quest structure at all (except for some minor side-quests), the location revolves around the tribal conflict, how you handle it is completely open-ended, you can kill the leader of one of the tribes to put the other in power, this isn't traditionally structured either, you just work straight in and open fire on Kebbekkah or Casey and the game responds automatically by "resolving" the quest. You can also broker peace between both tribes, but it's more tricky, requires more thinking, and the last "puzzle" to make peace is "hidden in plainsight".
Same goes for Titan Canyon, there's three ways to solve the central conflict by siding with either side, but there's also a "neutral" way to do it.
Also, you can open fire and kill any NPC in the game, so technically you could resolve things just by massacring people, the game supports it. That said, if you do round killing allies like in Highpool, Ranger Center will find out and call you to tell you what happened, some people said you came to their town and massacred people, which you can then say "We did it" or deny it.
I'm not sure how well that part of the game is developed, it might currently break the main quest and make you enemies of the Desert Rangers, BUT Brian Fargo has said that their will be a path in the Arizona map to turn against the Rangers by killing civilians, and then join some enemies of the Rangers, basically you won't get the full game, your party won't get sent to LA/California and the game ends prematurely (non-standard game ending), but the game will support.
I'm not sure how it'll work in California/LA, because it's unknown territory I doubt Ranger Citadel will be able to know if you commit atrocities, so you probably won't get booted, but there will probably be local consequences.