Some background:
A "shunt" placed across the rails will drop the signal circuit on which it's placed to "stop" or "stop and proceed". Conductors who are out working with maintenance-of-way crews actually carry these as standard equipment and use them to protect the work gangs.
If one is placed on the rails in front of an oncoming train (running with cab signals or other on-board equipment which enforces train speed), it will generally be forced into a "full service application" or penalty application. While this isn't a problem with passenger trains or short freight trains, on a LONG freight train it could cause problems with "the slack" in the train, ending up in a pull-apart (broken coupler knuckle or drawbar), perhaps even a derailment if a trains breaks in two and then the rear portion runs into the head end.
If no train is in the immediate area, it will drop the signals down, and trains approaching from a distance won't be severely affected.
Since this is a federal charge (in Washington state), the two may not get off so easily. If it was only "a state charge", betcha they would have pretty much "walked", after all, it's Washington state. However, considering who is coming to power in January, we'll have to wait and see...
