Subway Improvement Idea #3

Back in June, I posted an idea (or rather two) on how to improve the NYC Subways.  I still haven't found the MTA suggestion box, but I do have a NYC Subway Improvement Idea for July.  This time it's just a borrowed idea from my travels.  

Why can't we have LED signs that let expectant passengers know when the next train is expected to arrive?  Was it London that I saw them in?  Vienna?  Or just all of Europe?  And where can I find a photo of this sign so the blog can look all nice & pretty?