www.waspies.net wrote:

The vandals used acid to etch graffiti onto the trains windows, so many
were done that the service was suspended.


This is currently being discussed on uk.railway - thread title is "More
Graffiti".

The following text is taken from
http://www.vandalwatch.citysoup.ca/Graffiti/default.htm:

-----
Acid Etching

Acid etching is a form of graffiti that has started to show up in the
Lower Mainland. It is graffiti that is marked on glass or plexiglass
with an acid containing craft etching product, found in craft stores.
The graffiti is put on with the container. It will not show
immediately, but will burn into the glass over a short period of time.
The longer the acid etched graffiti remains, the more permanent it
becomes
The acid graffiti effects can be stopped or slowed down if the
substance is cleaned off soon after it is applied to the surface, or it
can be neutralized with water or baking soda paste. It is also possible
to buff the acid graffiti off of glass, but that must be done as soon
as possible. Acid graffiti is a costly problem in the Greater Vancouver
Region.
-----


Also see this New York Times article (with pictures):
http://tinyurl.com/3bnc7o