I think he's referring to graffiti on the engine, not Bad Order tags.

With the number of rail cars and engines that are left out in the open, in unsupervised areas, it is unlikely that you will see an untagged locomotive on the point. You would need to photograph a train that either has a brand new engine on the point, or a recently repainted engine, in order to find one that hasn't been tagged.

I hear the railroads take tagging very seriously (as they should). But with thousands of miles of unfenced and unpatrolled track, it is unlikely that it will be stopped anytime soon. Since it is not economical or feasible to fence off every mile of railroad track, nor would it be feasible to have every mile patrolled at all times, the best solution would be to create fines and penalties so strict that no one would want to be convicted of that crime. Since that is also unlikely to happen, and it's probably very difficult to prove who has tagged a particular locomotive or car, I unfortunately don't see this changing anytime soon. Just my 2-cents worth.