The original right-of-way and train tracks that Caltrain currently uses were built in the 1860s, before most cities and towns along the Peninsula were built, and especially long before the residential neighborhoods were built up along the train tracks. In other words, the railroad was here first. The original plan was to have four tracks wide all the way from San Francisco to San Jose. The two outer tracks were to be for local commuter trains and the two inner tracks were for express trains, long distance passenger trains and freight trains. Up until the 1980s, there were five tracks wide in a part of Redwood City.
Railroads all over America have been using a “set-out track” as common practice for the last 150 years. A “set-out track” is just that. It is a side track away for the main line where trains can set out a defective or broken down train car or locomotive, thus keeping the main line clear. At a later time, a rescue train would come and retrieve the broken down train car or locomotive and tow it back to the nearest repair shop. It is not a ‘RIP’ (repair in place) track. They are two different things.
I remember back when Southern Pacific Railroad ran the commuter trains in the 1970s-1980s. I would occasionally see a commuter train engine break down and the whole commute train would have to be either towed or pushed down the main line. This caused a huge backup of the following commute trains the rest of the evening. A set-out track is a necessary and standard practice for a railroad.