Fort Lincoln is the only Union presence in the Free State of California. While neither Shan Fan nor Lost Angels wants the fort to be here, both have a vested interest in selling Ghost Rock back east. The fort is also a thorn in the side of the Confederated States. The CSA has yet to put an official military presence anywhere in California.
The fort itself houses about 500 union citizens, mostly ghost rock miners. While the supply is small compared to Shan Fan or Lost Angels, its connection to the norther railway more than makes up for that.