Do the manufacturers of these aerosol sprays now liaise with bodies where their products are illegally used and produce special sprays that can remove or make easy to remove the graffiti. Most rail liveries are now vinyl-based and surely, it is not "rocket-science" to develop such products, with the technology available today.
What you tend to find is that once the railway (or any other grafitti removal company) finds an easy way of removing the graffiti, then the 'artists' change tactics and use a different type or spray with different chemicals.