Not in a practical place to provide useful citation, but I can’t recall any reference to indicate they were not part of the train and in fact, I swear I remember multiple references claiming they were in fact part of the train.
In my collections of plates (~4500 maybe, not exclusively French obviously), there are lots of depictions of “artillerie du train” and they are all in a pale or grayish-blue or “steal blue”. Facings vary within train troops with “artillerie” having a designated facing of dark blue pretty consistently. Engineers and others having each black and brown (can’t recall off the top of my head which for which).
Sorry I can’t be more specific at this time.