After the UP/SP merger BNSF was granted, and still has trackage rights from Richmond to Warm Springs (the trackage rights actually go all the way to San Jose). They ran a daily local from Richmond to Warm Springs for years to serve the NUMMI plant, so UP doesn't have any advantage in loading directly at the factory, as BNSF has access too.
The problem lies in the use of the yard trackage itself at Warm Springs. When Tesla took over the old NUMMI plant, UP retained the property that was used to load the autoracks, including any lots the cars would be stored in prior to loading. They (UP) tried to extort Tesla by giving Tesla two options, either a) ship with and be switched by UP for a low rate, or b) ship with BNSF and be charged an outrageously high rate to switch the ramps. They even went as far as to move all Newark and Milpitas based jobs to Warms Springs in anticipation of the new business. Tesla ended up going with option c) truck to Richmond and reload onto the BNSF.
The talks between Tesla and UP apparently got so bad that Tesla is allegedly making any new subcontractors sign a contract saying they won't ship with UP. UP has responded by completely tearing out what was left of the ramps at Warm Springs.
The contract between BNSF and Tesla has indeed already been signed, and Tesla cars have been getting trucked to Richmond for a few months now. Currently, an auto train is leaving Richmond about three times a week with about half of its load being Teslas. Work to convert the west end of Richmond yard to more ramps to load Teslas is already well underway. Once done, shipping is supposed to pick up to the 500+ daily Tesla cars that OP reported.