41 CFR § 301-10.190 - When may I use a transit system as a means of transportation in conjunction with official travel?
You may use a transit system as a means of transportation in conjunction with official travel when such transportation is authorized and approved by your agency in the following manner:
(a) At your official station.
(1) From your residence or other authorized point of departure, e.g., rail to airport;
(2) To your residence or other authorized point of return, e.g., airport to rail;
(3) From your residence to your office on the day you depart the official station on official TDY that requires at least one night's lodging; or
(4) From your office to your residence on the day you return to the official station from an official TDY assignment that required at least one night's lodging.
(b) At your TDY location.
(1) From the TDY transit system station(s) to your place of lodging or place of official business and return;
(2) To, from, and between your places of lodging and official business;
(3) Between places of official business; or
(4) To obtain meals at the nearest available place when the nature and location of the official business or the lodging at a TDY location are such that meals cannot be obtained there. You must attach a statement or include electronic remarks with your travel voucher explaining why such transportation was necessary.