Here's a reply I've found, though I haven't actually tested it, I believe it was from a Lionel person on their blog.
It is the data and com line, similar to the command base cable used to drive devices such as ASC/AMC/ARC, etc. The Data terminal is essentially a +5VDC communication line used to drive the old IC Controls line of devices, so a cable does not need to be run from the command base. If a command switch is in a close proximity to these devices you can use DAT and common to drive those devices.