Capturing all traffic between home network and cable modem

Is it possible to capture all WAN traffic (between a WiFi router and the cable modem) by inserting an Ethernet Switch and plugging in the router, Cable modem, and a PC running WireShark?

A switch would only work if it has a mirror port, so that you can see all traffic passing through the switch from that port. A true "dumb" hub (as opposed to a switching hub, which is a switch with the word "hub" in its description) would work. See the Wireshark Wiki's "how to capture on Ethernet" page for details.

This should be able to capture all traffic going from the machines on your home network to the cable modem or from the cable modem to machines on your hoe network.