I decided to do a search on the net for the symbol/logo that you've mentioned and this is what I found:
http://www.nyctransitforums.com/forums/showthread.php?t=2481
On No. 7 trains, red diamond means express, a green circle for local
BY PETE DONOHUE
DAILY NEWS STAFF WRITER
April 1st 2008
M. Roberts for NewsNo. 7 train riders looking to race from the subway platform to home plate at Shea Stadium should look for the diamond.
NYC Transit has begun rolling out subway trains with new digital signs brightly declaring if they are running express or local: a red diamond for express, a green circle for local.
The first train fully loaded with the broadsides hit the rails during last night's rush. More subway cars will be rigged in the coming weeks and months.
"It looks sharp," No. 7 line General Manager Louis Brusati said of the markings. "It will immediately tell people what train it is."
M. Roberts for NewsNYC Transit President Howard Roberts said: "It is another step in making the ride on the No. 7 line as smooth and effortless as possible."
Transit officials hope to trim train delays by making it easier to identify express and local trains. Service announcements are made on trains and in stations, but riders often are puzzled.
Straphangers regularly board trains before hustling back to the platform after learning they're on the wrong one. It's also common for someone to hold a train's doors open while asking those inside if the train is a local or express.
"That would be helpful," said Michel Vasiliou, a 31-year-old computer technician. "The drawback on the announcements is that sometimes they're not loud enough and you can't hear what train it is."
Audrey Molinare, a 28-year-old Queens nanny, also approved. "It's definitely a good idea if it would help with consistency," Molinare said.
Meanwhile, both No. 7 and L line riders will be the first to see a new transit communications strategy tested: digital signs or message boards in token booths and on station mezzanines. The wireless system will be used to tell riders about delays, route changes or other important information, Brusati said.
The first station in the six-station pilot may go online in about six or seven weeks, Brusati said.
The No. 7 and L lines are the first to feature a new management structure designed to increase accountability and improve service. Each line has a general manager instructed to run their line as if it's an independent railroad.
Images of the symbol/logo: