The mail platform, tucked away towards the southwest corner of the station, is a relict of Penn Station's past. During the height of the Pennsylvania Railroad's reign, huge percentages of the US Mail traveled by rail. A PRR train would arrive in Penn Station and crews would get to work pulling incoming mail cars off and adding outgoing ones on. After the train had left, the mail cars would then be corralled over to the Mail Platform, which was conveniently placed under the Farley Post Office across 8th Avenue, which did, and still does, process a huge amount of US Mail. Post office workers would then be able to get mail on and off the cars using the Mail Platform and various baggage elevators that went up and down from the upper levels to the platforms.
The platform was located over by yard E, and it's location is shown on the diagram below. The platform is a fairly long island platform, with tracks on both sides (it looks to be about 8 cars long). It is a high-level platform, as were all of the other station platforms in Penn Station.
Under today's current track configuration, the Mail Platform is not directly accessible from any of the tunnels into the station. In order to get over to the mail platform today you have to go down to station tracks 1 or 2 or over to yard tracks 1A or 2A by the Empire Tunnels and then reverse to get over to the platform.
Today, no mail arrives in Penn Station by rail, so no mail cars go over to the Mail Platform. The platform still exists, and it can be seen out the window of a NJT train pulling into track 1. When I was down there around the end of January, the platform was dimly lit and looked like it was being used for storage. I got a photo or two of it with my phone, but it was so dark you can hardly see anything.
Currently, the Mail Platform is just there fore storage. If I had to suggest a possible use for the mail platform, I would recommend that they add a crossover or two from the Empire Tunnel line down to the Mail Platform and use it to host Hudson Line trains from every Westchester County Politician's favorite project, Penn Station Access. They could have all the capacity on that platform and it would be just for them. They could link it into the new Farley Station Building since those two projects have the same level of unfundedness at this point., but that's just an idea.
In conclusion, the Mail Platform is just another relic of Penn Station's past. What was once a structure very useful to the Pennsylvania Railroad has turned into a place for Amtrak to store miscellaneous construction supplies. It would be nice to see mail come by train again and the Mail Platform would get some use, but that is a topic for another railroad and another day.