I did not find any "pass" or commuter type tickets available for shuttle flights on DL
and US. They basically price at about $145 each way if you book in advance. I did not see a "deal" on any IAD
service either. There are some discount fares on Acela if you book in advance and on specific train times but no pass. Regional Rail is not always significantly slower/worse than Acela other than seat type so take a look at schedules to compare. Acela can spoil you. You'll likely have to trim your travel home to fit in a travel budget since you live and travel to/from very expensive, high demand cities. D.C. is a dynamic city to live in and tons to do that is "free" as you discover living costs, but if you're from NYC you're already used to that. I starved being stationed in the military and then a low paid airline employee, but I left my heart and soul in Washington, DC. Always felt I was "home" and still feel that way decades later. A unique city. Good Luck with your new job!
watch what you want. you may get it.