>But what keeps the train moving? I know the answer to this question is inertia, but intuitively it makes sense that there must be some force that is making the object continue to move, even at a constant velocity. I guess a better question is do we know why objects with no net force can remain in motion? Like, it makes sense to me that when net force = 0 = no net movement, but not the constant velocity part.

Why is it that when you're standing inside a train (or airplane or car) moving at constant speed, you move along with the train without having to constantly horizontally push on the floor?

According to your reasoning, you're moving at constant velocity and that means you need to be pushing on something to keep moving forward. But actually you don't have to push on anything. Does that tell you anything about your intuition with respect to motion in different frames of reference?

You may also want to contrast this experience with your experience on something like a merry-go-round, where you know that unless you are actively exerting force against a pole or something else on the merry-go-round, you'll fall off. Do you know what the key difference between these situations is?