> it still shocks me that there were soldiers on my train in France with machine guns roaming the aisles.

It was probably their standard service rifles rather than machine guns. If it was around the mid 90s, the last three years, or some other period in-between with heightened terrorist threat, they were there for your security.

Unlike US policemen, French soldiers have never killed civilians until now, so you should feel quite safe when seeing them. They also don't arrest you, act aggressive or interact at all with civilians unlike border guards, so I don't see any link with the current topic.