Per your statement of "turning around, coming back and landing hard", it sounds like this was a return to launch potentially because your battery was low (it will warn you twice, audibly and visually as well). BTW, I would also just turn your volume all the way up, who knows maybe the train was too loud, but I do this so if I am watching the drone, I can hear the alerts or status. Anyway, I would like more details on exactly what happened, what the screen displayed at the time and see the video to be sure. Given this was your "first real flight", and another question you had on switching to easy mode, my guess is you have not updated it the Karma, so this also makes sense since you could not "regain control". The old update would return to home, but would not allow you to make adjustments via the sticks giving you the perception that you lost control. Even though you could not control a forced return via the sticks, you could cancel it per the X on the controller and regain full control. The auto return also lands harder than I would like, but if you launch area is flat, and you are not moving the drone around, then there is really no issue. I never allow an automated launch however, so the only time this would occur is in a really bad situation where I could not control the drone; such as a lost connection (never has happened to me), maybe I fell in a hole, or could not find the controller (yes, that happened once, so now I put a nice pink ribbon on the controller when I want to set it down to take an auto path of myself).
I would recommend reading the manual all the way though, even watching the instructional videos on GoPro's site. Careful of personal videos, as many are unfamiliar with the drone as well, so their perception, or interpretation of the issue is often incorrect. Either way, research will help you get familiar with the drone. When I first bought the drone, and even today especially after an update, I take it into a wide open area in the desert, far from anyone or anything and test it to actually see how the auto return functions be it self initiated, or letting the battery drain, even shutting off the controller. Early on I felt it was better to see what would happen while in a controlled environment, rather than at a time it would prove stressful. Either way, it matched both what the manual and instructional videos said it would do. I have never had an emergency in real flight, but if I did, there would be much less stress involved since I am very familiar with it, and how much juice it takes to get it home safely. Practice also helps me get better in flight and landing including accuracy, and landing by camera image which helps me since I take this on my boat as well, so return to launch is not going to work and I want to be able to land it anywhere at a moments notice.
Anyway, It's also important to update it. There are things I don't like about the newest update, but a big change was changing the default return altitude for 66' to 100'. From what I see, many never touch this setting, so it gets the drone a bit higher when a return home is initiated; hence stopping many from slamming into a building or mountain after they lost sight and flew behind them. The new update also gives you control of the sticks on a return, but to be honest I think that was a bad move as my son proved since less experienced pilots will try and control all the way to landing while RTL is still initiated. It is a hint tougher to land since the drone is landing itself with or without you, so it is common to touch down harder, or even float too much just before landing, or even bounce back up, which affords more room for error. From my view, if the pilot can still control it with the sticks, then why not take full control? If you can see the drone, then it's easier to fly around obstacles in manual mode. Now if there was a problem, like lost GPS, then maybe it helps, but then again it doesn't know where home is with lost GPS, so for most issues, I can't see its better to manipulate the drone when it is in auto return mode. Studies by GoPro probably found it was safer for the public to do it this way, and they really don't care about damaging the drone. So, I get it; I just will never use it unless I have no choice.