USB Power Bank
One of the lessons that I learned from using the commuter rail during last year's winter is to always carry a fully charged USB power bank. Not only has this has come in handy when waiting for the train but also for those delays while on the train.
Here's What Happens
When it's freezing cold the iPhone can go into shut off mode. Yes, this happened to me several times. There were times when I would be using the phone on a really cold day and then without warning the phone would shut off. The iPhone 6s Plus operating temperature is above freezing (32 degrees or 0) This means that the phone will shut off when the temperature reaches the extreme temp. I have found it happens when the "feel like" temperature is 0.
As the temperature gets colder, the phone battery charge doesn't hold as well. This means that even if you don't use the phone, the extremely cold temperature will actually shorten the battery charge. I encountered this issue also.
Power of the USB Power Bank
This is why having a USB power bank is critical to carry in your laptop bag. In addition, I find keeping a small cable with the power bank makes big the difference. I always know that the cable is there.
On eBay you can get a small 8" flat Braided cable for about $5. This can help justify having a "just in case" cable. What's more, you can get different tips for different devices - for example if you want to charge your Kindle or camera.
I recommend getting a short length cable so it doesn't take that much space in your bag. I find that the 8 inch is the perfect length.
I purchased two cables in different colors. I have a red one for my phone and a green one for my Kindle. I select red as important, as my phone is pretty important. I selected because that was the color of Amazon's logo.
How the USB Made a Difference
Last winter, on some of the coldest days, I found that when I got on the train that I had to energize the phone by connecting the phone to the USB power bank for about 5 minutes. Once it got enough juice, the phone was back to normal.
The iPhone 6s Plus battery size is 2750 mAH. So it's best to get a USB Power bank at least 5000 mAh so that you have enough charge for those long delays.