It depends on the amount of light available and the field of vision from the cab, but drivers should be alert to their surroundings and I would certainly expect there is a high chance they would see something like a wood chipper on an adjacent line. If so, the actions taken would really depend on what it was deemed to be (not always obvious) and what needed to be done, but realistically I would expect the driver to stop their train and report it urgently to the signaller. It all depends what was going on in the driver's surroundings and whether the obstruction was in fact on the line at the time, though.
There are various other options such as the use of flashing hazard lights on some stock, switching headlights to red, etc., but if the train has not actually hit an object and is not causing an obstruction in and of itself, it may not be deemed necessary to use those options, especially if the signaller can be readily contacted.
