I see your point, Dear Reader elmore 3003. Still, there's that nasty bout of singing the poor thing needs to do in Act IV. But, hey, it's opera.
On the other hand, they agree to make up and be friends. She later takes up with a wealthy man and is seen riding through the streets in a fine carriage. Then, when she's so sick she gets dumped, she runs back to Rodolpho.
I say she was quite well enough to take a train to the French/Belgian border. It's not like she WALKED all the way.
Besides, her illness waxed and waned. And more importantly, she didn't know she was dying. She might very well have taken a train to find her great love.
The location of where that scene is set shows that Rodolpho was serious about getting away from her -- that she maddened him so much he needed to put some distance between them. The gates of Paris wouldn't be that much distance.
The bottom line: The music is not changed by a single note, and as Jay says, it's OPERA! Please don't let's start talking about holes in plots of operas.
It's a glorious production. I saw it in San Francisco, too, Jay! It was one of the seminal theatrical experiences of my life.