Changes

* ''At crossings'' to prevent train from blocking the crossing, if the train cannot continue beyond it. Correctly placed signals are '''Chain before''' the crossing and '''Block after''' the crossing. If the track after the crossing is empty -> the block signal is green -> the chain signal before the crossing is green. If the block after the crossing is occupied -> the block signal is red -> the chain signal before the crossing is red as well so next train won't enter the crossing thus it will not block the traffic on the other track. For instance, imagine the upper left track leading to an off-screen loading station, and the first train is waiting its turn. The second train will not enter the crossing (and block it) while it waitsbehind the first train, thanks to the chain signal.