I think I have the answer here, tho. Higher capacity railway cars, more powerful locomotives, larger freight cars, especially taller ones meant you could get away with single track + sidings. Tunnels that once let shorter (top to bottom, not front to back, but that, too) trains pass each other along two tracks now were too short to let the tall trains go through. But if you ripped up the tracks and put one down the middle, the arch let you get substantially more height through the tunnel.
Voila. Single track everywhere. Also, the myth of overcapacity. Well, yeah, if you built your system to last forever and ran stuff size X on it, and then started assuming the system could be shredded and ran stuff size XXXX on it, it would definitely _seem_ like it used to have overcapacity. That's right up there with a financial system that, say, requires reserves for banks, or doesn't let savings and loans act as investment banks, or requires home buyers to put up 20% being compared to one that lets the banks use dog turd tranches as collateral, and any bank can do anything, and anyone can buy any house for no money down -- the second one _definitely_ supports a larger economy and the previous one seems overly conservative and doesn't generate hardly any construction jobs.
For a while.