Hm, that my post was really not very accurate... What I can find in fan sources CivII Superhighways do:
1) Any Road/Railroad city tile +50% Trade; https://civilization.fandom.com/wiki/Superhighways_(Civ2)
2) +50% trade route output for either end (i.e. +100% if SHW in both ends). http://www.codehappy.net/apolyton/threads/43911-1.htm Though https://stefan.winkler.site/games.html suggests only home city SHW affects the output, I don't have CivII to test...
Alexandro Ignatiev in https://www.hostedredmine.com/issues/889197 :
"SuperHighways effect from CivII that, alongside with city trade increment, also gave bonus to trade routes' output in its city (but not in its trade partners)."
We should add that effect to civ2 ruleset. Just needs to figure out what value it should have.