uhm, yes, of course that opto is supposed to work?

if it's really just one opto and you know which one, then it might be easier to reflow the solder and clean it off thoroughly. if your subway is dirty, the optos will continue to build up dirt fast, though.

Yeah, it is an important opto. It is obviously causing the failure you are seeing.
I suggest replacing all the transmitter optos on the trough transmitter board. Transmitter optos are on all the time when the game is on and after 20 years, they all will die at some point. Also you would want to check all the other solder joints on the board as well for any cracked solder joints.

http://www.mouser.com/ProductDetail/Fairchild-Semiconductor/QED123/?qs=KIIz7VjqD7I4A%252bR7oPm1DQ==

or

https://www.greatplainselectronics.com/search.asp
search for p/n QED123

Sometimes the lead on the load resistor for the LED will break from all the vibration this area sees, could be a real easy fix.

I was able to repair a similar issue. I reflowed the solder joints on the transmitter board. The resistor solder joints were cracked.

Did you order the trough opto set from Great Lakes modular ? I had a lot of problems with the original boards and I replaced both trough boards and there connectors an no more issues

I had posted the info on the opto parts. Not wanting to replace them? Only would had been $8 with shipping.