>>2058163 (OP)
I assume because they can not guarantee that the graf artist hasn't fucked with the train in another way (severing lines, tampering with equipment etc) so they need to clean and inspect before being put into service. I've never tampered with a train after painting it but some wilder crews will intentionally break shit.
I love metro graf. unofficial, unpaid, illegal penetration testers that show how pathetic modern day metro security is. Imagine having 30+ mins of unrestricted access to mass public transport carriages if you were a bad apple. Unthinkable