an "annual deduction?"  Does that imply they live in DC year round?

Home is in Madison MS and work and live in Washington 250 days a year

DC is probably their tax home. Only Congress gets that special deduction for maintaining a second home in DC.

In other words, travel deductions generally don't apply if they are away from "home" (their personal residence and family", the deductions apply if they are away from their "tax home" (usually their main place of work).

I believe, for 2018, their is no deduction because those expenses would be part of Misc Itemized Deductions subject to the 2% floor and those have been eliminated.

Dusty Ernie