was wondering if it's possible to switch from Pacific Rail (west) to Atlantic Rail (east)?
One would have to lay tracks down to one of the eastern cities.
No, and No.
Once you are aligned with Red, or Blue, you are that color for the duration of the game. Yes you can track to a city of a different color, but your color will not change even if you change your home. That is the point of Red VS. Blue. IF you are Red, and connect to a Blue city, your goods will count towards the Red percentage to take over the city.
Steamy was right though, you can change by selecting a different city at the start of a new round, but during the round you cannot change.