Vandals used to spray paint the glass and plastic surfaces inside the trains. This led the MTA to install plastic coated glasses on windows and metallic surfaces on the walls to enable easy and inexpensive cleanups. The vandals are now using acids to burn through the plastic coatings and scorch the surfaces beneath them. That is why a lot of the new graffitis have drip marks on them.
I took this photo on my way home from work last Friday. You can see the signature/symbol of the vandal etched in acid on the train's window. I hope the MTA is successful in prosecuting these dumb**ses for defaceing public properties.