I just completed Blind Betrayal for the BoS, and was by impressed. So much so, that I now want to defect to the Railroad. However, I was required to turn the Network Scanner Holotape in to get this far, and I'm not sure if I have the option to switch factions at this point. Would I be able to start helping the Railroad at this point, or am I locked into the BoS?

You're still able to get the Railroad as long as they're not hostile and you haven't become hostile against the Institute. The network scanner holotape doesn't matter at all. You're not forced to make a faction hostile until a quest called Mass Fusion.

Okay. Thank you. As I said, I was really impressed with the Blind Betrayal quest. I'd been thinking of switching factions, but had ultimately decided I wanted to see how this quest played out before I made the final decision. And the guide makes a huge deal about the Network Scanner Holotape, but doesn't seem to say anything about the actual Point of No Return, so I thought I might be stuck with the Brotherhood.

Yeah, no problem. It seems to be a common rumor/misconception that your choice for the Network Scanner Holotape matters in the long run, but it definitely doesn't. I saw people answering incorrectly when the game first released, so I'm assuming that's where the misconception got started. The guide you're using probably relied on those responses or made an incorrect guess.

So, it turns out that I did actually lock myself out of doing the Railroad quests. Mainly because I took the time to talk to meet Father and talk to the department heads. As far as the Railroad is concerned, that means I'm working with the Institute, so they won't work with me. I'm not sure if only talking to Li for the BoS quest, then grabbing the data and leaving is okay, because otherwise I don't see any way to swap from the BoS to the Railroad, and all of a sudden, I'm very attached to that RP.

Did you join the Railroad prior to actually going to the Institute because if you were already a member prior to getting the courser chip, you can still do their quests. If you didn't, doing even ONE quest for the Institute locks you out of the Railroad.

I didn't. I was working with the BoS, and figured I'd be able to switch with no problems. The only reason I was able to get into the Railroad HQ was because they have to decode the Courser Chip.