Well, they kept on coming so I turned on a packet sniffer and saw one connect to:
The DNS wouldn't do reverse resolution on that IP address for me.
But the SSL certificate on that server identifies it as Qualcomm Omnitracs for mcp200-ssl.omnitracs.com.
Connecting to it gets me to a Cisco VPN login page.
Google says Omnitracs is a fleet management company that qualcomm spun off. So it looks like my guess was right - these are on vehicles, probably delivery trucks.
To answer chmcwill's question: I am about a mile from the nearest railway but I do have line-of-sight to it.