The Roman subway trains are covered in graffiti in a way reminiscent of the New York City subway trains in the 1970s and 80s. I assume the yards where the trains are parked at night are insecure – that would be the only explanation I can think of for the sheer amount of graffiti.
While my memory of New York City subways in the 1970s is a bit vague, my impression is that the graffiti there was not so much artistic as it was mostly tagging by individuals and/or gangs. Here, as in seemingly all things Italian, there is an underlying artfulness to at least some of it.
And even the text commentary (“Who sleeps not [something] with the fishes”) is relevant to the designs. I don’t know if my misreading of the handwriting is wrong or if the phrase is some Italian/Roman slang phrase that Google Translate can’t figure out. Any readers who understand the expression, please chime in and correct me!