Program: 171555
Project: 171555
Buggy source:
@Override public void onUpdate(){
  super.onUpdate();
  if (world.isRemote) {
    return;
  }
  if (this.getCurrentSpeed().minecraft() != 0) {
    ChunkManager.flagEntityPos(this.world,this.getPosition());
    if (this.lastKnownFront != null) {
      ChunkManager.flagEntityPos(this.world,this.lastKnownFront);
    }
    if (this.lastKnownRear != null) {
      ChunkManager.flagEntityPos(this.world,this.lastKnownRear);
    }
  }
  for (  CouplerType coupler : CouplerType.values()) {
    if (!this.isCoupled(coupler)) {
      continue;
    }
    EntityCoupleableRollingStock coupled=this.getCoupled(coupler);
    if (coupled == null) {
      if (this.ticksExisted > 20) {
        ImmersiveRailroading.warn("String_Node_Str");
        this.decouple(coupler);
      }
      continue;
    }
    if (coupled.isDead) {
      ImmersiveRailroading.warn("String_Node_Str");
      this.decouple(coupler);
      continue;
    }
    if (coupler == CouplerType.FRONT) {
      lastKnownFront=coupled.getPosition();
    }
 else {
      lastKnownRear=coupled.getPosition();
    }
    if (!this.isCouplerEngaged(coupler)) {
      CouplerType otherCoupler=coupled.getCouplerFor(this);
      if (otherCoupler == null) {
        continue;
      }
      if (this.getCouplerPosition(coupler).distanceTo(coupled.getCouplerPosition(otherCoupler)) > Config.couplerRange * 4) {
        this.decouple(coupled);
      }
    }
  }
  for (  CouplerType coupler : CouplerType.values()) {
    if (!this.isCoupled(coupler)) {
      Pair<EntityCoupleableRollingStock,CouplerType> potential=this.potentialCouplings(coupler);
      if (potential == null) {
        continue;
      }
      EntityCoupleableRollingStock stock=potential.getLeft();
      CouplerType otherCoupler=potential.getRight();
      this.setCoupledUUID(coupler,stock.getPersistentID());
      stock.setCoupledUUID(otherCoupler,this.getPersistentID());
    }
  }
  if (world.isRemote) {
    return;
  }
  if (this.getRemainingPositions() < 20 || resimulate) {
    TickPos lastPos=this.getCurrentTickPosAndPrune();
    if (lastPos == null) {
      this.triggerResimulate();
      return;
    }
    if (!(this instanceof Locomotive)) {
      for (      EntityCoupleableRollingStock stock : this.getTrain()) {
        if (stock instanceof Locomotive) {
          stock.resimulate=true;
          return;
        }
      }
    }
    boolean isStuck=false;
    for (    EntityBuildableRollingStock stock : this.getTrain()) {
      if (!stock.areWheelsBuilt()) {
        isStuck=true;
      }
    }
    Speed simSpeed=this.getCurrentSpeed();
    if (isStuck) {
      simSpeed=Speed.ZERO;
    }
    this.positions=new ArrayList<TickPos>();
    positions.add(lastPos);
    for (int i=0; i < 30; i++) {
      if (!isStuck) {
        simSpeed=this.getMovement(simSpeed);
      }
      TickPos pos=this.moveRollingStock(simSpeed.minecraft(),lastPos.tickID + i);
      if (pos.speed.metric() != 0) {
        ChunkManager.flagEntityPos(this.world,new BlockPos(pos.position));
      }
      positions.add(pos);
    }
    simulateCoupledRollingStock();
    for (    EntityCoupleableRollingStock stock : this.getTrain()) {
      stock.resimulate=false;
    }
  }
}

Fixed version (ground truth):
@Override public void onUpdate(){
  super.onUpdate();
  if (world.isRemote) {
    ParticleUtil.spawnParticle(world,EnumParticleTypes.REDSTONE,this.getCouplerPosition(CouplerType.FRONT));
    ParticleUtil.spawnParticle(world,EnumParticleTypes.SMOKE_NORMAL,this.getCouplerPosition(CouplerType.BACK));
    return;
  }
  if (this.getCurrentSpeed().minecraft() != 0) {
    ChunkManager.flagEntityPos(this.world,this.getPosition());
    if (this.lastKnownFront != null) {
      ChunkManager.flagEntityPos(this.world,this.lastKnownFront);
    }
    if (this.lastKnownRear != null) {
      ChunkManager.flagEntityPos(this.world,this.lastKnownRear);
    }
  }
  for (  CouplerType coupler : CouplerType.values()) {
    if (!this.isCoupled(coupler)) {
      continue;
    }
    EntityCoupleableRollingStock coupled=this.getCoupled(coupler);
    if (coupled == null) {
      if (this.ticksExisted > 20) {
        ImmersiveRailroading.warn("String_Node_Str");
        this.decouple(coupler);
      }
      continue;
    }
    if (coupled.isDead) {
      ImmersiveRailroading.warn("String_Node_Str");
      this.decouple(coupler);
      continue;
    }
    if (coupler == CouplerType.FRONT) {
      lastKnownFront=coupled.getPosition();
    }
 else {
      lastKnownRear=coupled.getPosition();
    }
    if (!this.isCouplerEngaged(coupler)) {
      CouplerType otherCoupler=coupled.getCouplerFor(this);
      if (otherCoupler == null) {
        continue;
      }
      if (this.getCouplerPosition(coupler).distanceTo(coupled.getCouplerPosition(otherCoupler)) > Config.couplerRange * 4) {
        this.decouple(coupled);
      }
    }
  }
  for (  CouplerType coupler : CouplerType.values()) {
    if (!this.isCoupled(coupler)) {
      Pair<EntityCoupleableRollingStock,CouplerType> potential=this.potentialCouplings(coupler);
      if (potential == null) {
        continue;
      }
      EntityCoupleableRollingStock stock=potential.getLeft();
      CouplerType otherCoupler=potential.getRight();
      System.out.println(this.rotationYaw);
      System.out.println(stock.rotationYaw);
      this.setCoupledUUID(coupler,stock.getPersistentID());
      stock.setCoupledUUID(otherCoupler,this.getPersistentID());
    }
  }
  if (world.isRemote) {
    return;
  }
  if (this.getRemainingPositions() < 20 || resimulate) {
    TickPos lastPos=this.getCurrentTickPosAndPrune();
    if (lastPos == null) {
      this.triggerResimulate();
      return;
    }
    if (!(this instanceof Locomotive)) {
      for (      EntityCoupleableRollingStock stock : this.getTrain()) {
        if (stock instanceof Locomotive) {
          stock.resimulate=true;
          return;
        }
      }
    }
    boolean isStuck=false;
    for (    EntityBuildableRollingStock stock : this.getTrain()) {
      if (!stock.areWheelsBuilt()) {
        isStuck=true;
      }
    }
    Speed simSpeed=this.getCurrentSpeed();
    if (isStuck) {
      simSpeed=Speed.ZERO;
    }
    this.positions=new ArrayList<TickPos>();
    positions.add(lastPos);
    for (int i=0; i < 30; i++) {
      if (!isStuck) {
        simSpeed=this.getMovement(simSpeed);
      }
      TickPos pos=this.moveRollingStock(simSpeed.minecraft(),lastPos.tickID + i);
      if (pos.speed.metric() != 0) {
        ChunkManager.flagEntityPos(this.world,new BlockPos(pos.position));
      }
      positions.add(pos);
    }
    simulateCoupledRollingStock();
    for (    EntityCoupleableRollingStock stock : this.getTrain()) {
      stock.resimulate=false;
    }
  }
}