When biter "Unit groups" in the debug info display reaches into the many hundreds and thousands and the biters are standing around not attacking, this will clear the units and update the fog of war to get rid of stale entries.
/c game.forces["enemy"].kill_all_units() --[[ not a semicolon --]] game.forces.player.chart_all()
Start all the trains in a given area:
/c for k,v in pairs(game.player.surface.find_entities_filtered({name='locomotive', area={{x1,y1},{x2,y2}}})) do v.train.manual_mode=false end