It was Waylon Jennings instead of REO Speedwagon, but that was okay with Tim. Waylon was followed by Shooter Jennings and Marty Stuart. The two men in the mud-streaked Dodge Ram listened and watched the highway roll. Seventy miles up the line, the old guy pulled over, gave Tim a tip of his Case cap, and wished him a real fahn day.
This section is from the book, The Institute by Stephen King, chapter 2. What does "up the line" mean here? Thanks for your help!
As Kate alluded to in a comment, "up the line" or "down the line" usually refer to a railroad line. The meaning is: some [possibly specified] distance along the route. Whether "up" or "down" is used depends on context (elevation, direction relative to a large city, direction relative to the train's direction) and the words are essentially interchangeable. (The terms could also refer to other "lines," like a telephone or Internet communication line.)
The usage here applies the term to a truck on a highway, which is strange—I would have expected "down the road" rather than "up the line." In any case, "seventy miles up the line" means "seventy miles away by road." Probably this means seventy miles from where Tim started riding with "the old guy."
Here, "line" likely means "route".
Trains run along lines because that's where the tracks are. Buses have "lines" because they always go the exact same route. Regular trucking routes are also often referred to as "lines", as in the name of the major truck brand, "Freightliner".
From Merriam-Webster, with my highlighting:
6 h (2) : a system of transportation together with its equipment, routes, and appurtenances
the eastern freight lines
also : the company owning or operating it
