This is a huge feature for people who like to walk in NYC, as I do. 

Now when you click on a blue M, which is the symbol for a subway station, it hightlights the route of one of the lines going through that station.  

For example, I clicked on the High Street/Brooklyn Bridge station, and it showed the route of the A and C lines in blue.  

Click on Grand Central and it shows the routes for the Lexington Ave lines, the 4, 5 and 6. And the Flushing line (the 7). 

This is a very important connection. Not sure when it came online, but all NYers and esp people who visit, will want to know. 

PS: Same with the Bay Area, Boston, Chicago.