You will not be forced to any terminal other than the one you hire out of as a conductor with BNSF. You may not be able to hold a position at the terminal you hire out of, so you may then choose to go to another terminal to work. At terminals where BNSF is short manpower, lodging and meal pay may be offerred to entice crew from other terminals to work there. BNSF conductors have system-wide seniority, excluding the territory roughly from Minnesota to Washington, so if you hire out of Phoenix, you can work in Chicago, Houston, Denver or Kansas City, among many other terminals, once you are qualified at the new terminal. Getting qualified at the new terminal is called familiarization, which consists of taking a prescribed number of trips and working a series of yard/local/road switcher jobs to experience the new territory. Depending on need at the terminal and your ability to hold a job at your regular terminal, you may or may not be paid to take these familiarization trips.
The one instance of forcing I am aware of (for conductors anyway, it's different for engineers) is that some terminals will hire conductors under a Primary Recall agreement, which stipulates that for the term of the agreement (5 years), should manpower needs dictate, you can be recalled to your home terminal from where you are currently working. For example, you hire out of Houston under a Primary Recall agreement. After six months, you can't hold a job there, but you can in Oklahoma City. So you move to OKC and begin working for 4 years without a problem. Traffic picks up in Houston while you are away and suddenly conductors are needed there. You receive a notification that you have been recalled to Houston and must report to work there in 72 hours. Until the manpower situation is taken care of by new hires or reduction in traffic, you will be held to your recall terminal.