Do you have a good reference for that?

Grid operators in general don't like that; they want something to buffer the feed back to the grid. Either that, or the trains have to rely entirely on friction braking (air brakes on Caltrain, most likely) in emergencies. An uncontrolled feed back to the grid is one of the things that has triggered the end of net metering - grid operators don't want to be forced to accept power when they don't need it.

LRVs that are all-electric (as are most in the western US) must have resistor grids for that reason. It would be odd for Caltrain to be much different.