it is a little bit of both.
before graffiti writers realized their work would get painted over if the reporting marks were covered yes, they painted everything. initially, graffiti writers would re-write the serial numbers back with the same aerosol they painted their pieces with. they quickly realized the rr would not spare their work even if they did that. so around the turn of the century, writers began to either avoid the numbers all together, carefully paint around the numbers, or use masking tape to mask the numbers off, do their work, take a photo and then remove the tape. there are still some however, that paint over the reporting marks knowing their work will get painted over. even on the NY subways back in the 70s and 80s, writers painted over the windows knowing full well that part of the train would get cleaned immediately.
I am not employed by the rr so cannot speak factually to rr companies position on graffiti. what I DO know for sure is they don’t like it. how reporting marks are repainted probably vary greatly on who is doing the repainting. ive seen some just stencil the numbers back over the graffiti so the numbers are visible again, while others paint huge squares and then repainting the numbers so they are even more clear - the latter being the most common method. A rail worker told me years ago that having reporting marks exposed on all sides of any car that is in service is a legal requirement. failure to comply could result in DOT penalties for the railroad. so the reporting marks, light/load limits and COTS box are almost always re-exposed, swiftly, to avoid these penalties. railroads have tried a number of tricks to avoid coverage by graffiti writers. one of the most clever is newer cars have the reporting marks painted high up on the car. QGRY, NCAX, and the CN family of roads have been doing this for years. but i have seen more roads trying this recently.
as for commissioned rr graffiti, that’s a fine idea. Although, I think there would be two very general challenges with that idea:
1) the NY transit authority still vehemently opposes graffiti. According to their records, graffiti on the subway of New York City never existed. it is not included in any public history recorded by the organization (museums, books, websites etc.). I would imagine rr companies have the same or similar position. they do not want to do ANYTHING to promote, or give the impression of promoting graffiti. they learned from the NYCTA (and a little common sense) that promoting graffiti, in any way, would be counter productive to the goal of preventing graffiti.
2) as a society we just are not there yet. it would probably take another 2 or 3 generations before graffiti would be viewed as an art form within the rr community. just reading this forum and others, it is clear that the rr community at large views graffiti as vandalism - period. the artistic value is never even considered because the bottom line is the graffiti writer is defacing private property, thats all. any varying opinion or sympathizer is usually called a bleeding heart liberal or the like. It often results in a heated debate that winds up getting the thread closed. Again, calm rational dialogue, respecting varying opinions and positions is a station we simply have not pulled into yet.
Cheers~