The only times I've seen the LED signs on the platform display anything other than something like "This is [name of stations]. Watch for trains" is when a "Voice of Metra" announcement is made when they display a text version of the announcement. There is no continuous updating of the time to next arrival.
As far as I can tell, the announcement of the next train is triggered at a fixed point ahead of the upline station. I've noticed at Roselle that the next inbound train announcement says 7 minutes for most trains but for the one I take most mornings, it says 8 minutes. But most trains are allowed five minutes from Schaumburg to Roselle while my morning train is allowed six minutes, hence the extra minute (my guess is the announcement at Roselle occurs at the same time as Schaumburg gets the "now arriving" announcement. But I don't hear the announcement all that often - I have my trip to station well timed enough that I'm rarely there more than five minutes before.
If I ever have a chance, I should go out there early and hear the announcements for 2208 at 6:26am (express Elgin-National St. to Roselle) and 2212 at 6:54am (deadhead from Elgin to originate Roselle). Also I know from experience that 2220 (deadhead from Chicago, reverse west of Roselle to originate Roselle - 7:37am - this was my regular train until about two years ago) gets no "arriving in X minutes" announcement and because where they reverse is east of where the "now arriving" message is triggered, as soon as they set the GPS up for the trip, the "now arriving" message plays immediately, even if they are not ready to go. On more than one occasion, they set it up before 2218 (does not stop at Roselle) had passed resulting in the "now arriving" message playing as 2218 was approaching the station at 70mph (not good at all).
Reno, NV (yes, I've moved)
ex-N&W Sandusky, Ohio