RS-232, common ground, keep it under 2400 baud, and I can see that working just fine.
Your geek gets a 
from me for that one… 
10 Mb/s Ethernet would go down wet string, I have done some appalling things involving leads with alligator clips and 10Base5. It would not surprise me that a line of paper-clips would work.
Have you actually tried the wet string?
No sadly. I doubt it really would work, too high a resistance. Maybe salty wet string… It would be a fun experiment, I doubt I have anything left that would boot that supports 10Base5. I had a couple of 5-2 adaptors, but I scavenged them for the boxes and power supplies.
My users let it dry out. :rolleyes:
Up until just a few years ago, many subway (technically, “electric railway”) cars used motors and control system designs that date back to the introduction of subway cars. The materials might be different (modern plastics vs. Bakelite, aluminum wiring instead of copper, etc.) but the principles were exactly the same. In fact, if you could travel back in time and collect a train operator / engineer, they would be right at home with nearly present-day equipment. And their tools (brake “handle”, various keys) would likely still work as well.
This is an example of the “installed base problem” - there’s so much equipment out there using the older types that changing over would be prohibitively expensive. There have been recent changes to improve efficiency, like AC motors instead of DC, regenerative braking, etc. but the power supplied to the cars is still 600-750 Volts DC and each car converts it to AC.
The non-moving parts of transit systems tend to get upgraded technology, since it is possible to change a small part of the system at a time. So things like signals are being upgraded from 25Hz AC to modern power, with fiber optic communication links instead of bulky cables, and LEDs instead of incandescent lamps with obsolete base types.
Lead Acid Battery - 1859
Edison Lamp - 1878 (Others claim earlier invention, but his basic design is the one still around)
The Dry Cell (Common Zinc-Carbon) Battery was 1886
Excellent Book - “The Battery” by Henry Schlesinger