public ExtendRailAction(RailNode prev, RollbackFn fn)
 {
     prevTail = prev;
     rollback = fn;
 }
 public CreateLineAction(ModelStorage db, ModelListener lis, RailLine line, RollbackFn fn) : this(db, lis)
 {
     prevLine = line;
     rollback = fn;
 }
 public StartRailAction(ModelFactory f, RailNode prev, RollbackFn fn)
 {
     factory  = f;
     prevTail = prev;
     rollback = fn;
 }
 public BuildStationAction(RailNode tail, Platform tailP, RollbackFn fn)
 {
     prevTail     = tail;
     prevPlatform = tailP;
     rollback     = fn;
 }