The letter doesn't support that the prosecutors are not following up and prosecuting these criminals, rather it says the exact opposite.
Rather, the complaint seems to be that the government is granting the criminals their constitutional rights to bail and a presumption of innocence before trial, that the prosecution is accepting milder plea deals than the railroad company would like, and that the court system is backed up.
Emphasis added to the below quote to support each of those statements
> Criminals are caught and arrested, turned over to local authorities for booking, arraigned before the local courts, charges are reduced to a misdemeanor or petty offense, and the criminal is released after paying a nominal fine. These individuals are generally caught and released back onto the streets in less than twenty-four hours. Even with all the arrests made, the no-cash bail policy and extended timeframe for suspects to appear in court is causing re-victimization to UP by these same criminals.
This is the railroads side of the story, and even it doesn't support the idea that the prosecutors are just "not follow[ing] up and prosecut[ing] these criminals".
Moreover the court system being backed up is not prosecutors fault, and not offering mild plea deals would just result in a more backed up court system...
There are probably some valid criticisms of the prosecutors here, but let's at least consider that there are tradeoffs they are making, and that this isn't something that is solely their fault.