Who Says “The Amtrak”?

I once met a man who could listen to you speak for less than a minute, then tell you where you grew up. And he was very specific. He didn’t say, “You grew up in New England”. He was much more specific. He told me, “You grew up in Connecticut, probably somewhere northwest of Hartford, up near the New York border.”
The fact is, I grew up 15 miles northwest of Hartford and about 40 miles from New York State. Yeah … he was really good.
I’ve always been fascinated by regional accents and how there can be a pronounced difference in the way people talk even when they live only a few miles from each other. Perfect example: the classic Boston accent (Pahk ya cah in Hahvahd Yahd”) compared to the flat, clipped speech of a “down easter” (which defies a phonetic spelling) living just 30 or 40 miles up into New Hampshire.
And, depending on where you live, everyday items have very different names. Take, for instance, the ubiquitous submarine sandwich. As far as I know, it’s simply called a “sub” most everywhere. But not in New England—or at least not in Connecticut where I grew up. There, it’s a “grinder”.
And consider that delicious soda fountain treat I grew up with: a couple of scoops of ice cream and a cup or so of milk, all whipped up in a blender. It’s called a milkshake, isn’t it? Well, not in Boston. There, it’s “a frappe”. And note, please, it’s not pronounced frappé, as with that fancy accent aigu over the “e”. In Boston, it’s a FRAP!
All of which prompts me to ask why, and in what parts of this country, do some folks always refer to “the Amtrak” . . . as in, “I’m going to take the Amtrak up to Seattle.” For reasons I cannot explain, that jangles my senses. When I travel, I take Amtrak, not THE Amtrak.
Does anyone know where that came from? Is it generational? Or regional? Curious, isn’t it. And fascinating, too.