Hi!
I transitioned while working for the US Forest Service. I was hired as female and now work full time as male. I announced my intention to transition a few days after I was officially hired.
Bottom line: everyone was GREAT
As soon as I notified my supervisor what I intended to do, the regional civil rights director was on the phone with me, making sure things were ok and opening up a line of communication should I have any issues.
My immediate coworkers and higher ups were notified (I didn't have to do any of that) and from there on out it was "he-" and "him-" across the board. Bathrooms at my workplace are single occupancy, so that was never an issue. I used the men's rooms from day one. Once I had my legal name/gender change, the office administrators were thorough and discrete about getting all my official paperwork updated.
These days, 99% of my fellow employees think I'm just another male. Even my cis male office mate (who I know knows) could fool me with how readily he engages me in 'guy talk'. No weird vibes, even once people found out I'm gay (and a lot of my coworkers are grizzly adams forester/trapper types, ha).
It's really been a great experience, and it gives me hope for all future transitioners in US fed. service. Just remember your rights! Most likely try very hard to accommodate you, because no gov organization wants negative press regarding LGBT folks these days...
Good luck!