diff --git a/src/edu/kit/informatik/model/ModelRailwaySimulation.java b/src/edu/kit/informatik/model/ModelRailwaySimulation.java
index 2fc501d..c75eaa9 100644
--- a/src/edu/kit/informatik/model/ModelRailwaySimulation.java
+++ b/src/edu/kit/informatik/model/ModelRailwaySimulation.java
@@ -76,10 +76,7 @@ public class ModelRailwaySimulation {
      */
     public boolean removeRail(final int id) {
         // check whether any trains are on this rail
-        if (trains.values().stream().anyMatch(train -> train.isPlaced() && train.isOnRail(id))) {
-            return false;
-        }
-        return railNetwork.removeRail(id);
+        return trains.values().stream().noneMatch(train -> train.isOnRail(id)) && railNetwork.removeRail(id);
     }
 
     /**
@@ -111,15 +108,9 @@ public class ModelRailwaySimulation {
      */
     public void createEngine(final Engine newEngine) throws InvalidInputException {
         String id = newEngine.getIdentifier();
-        for (Engine engine : engines) {
-            if (engine.getIdentifier().equals(id)) {
-                throw new InvalidInputException("engine identifier already used by engine");
-            }
-        }
-        for (TrainSet trainSet : trainSets) {
-            if (trainSet.getIdentifier().equals(id)) {
-                throw new InvalidInputException("engine identifier already used by train set");
-            }
+        if (Stream.concat(engines.stream(), trainSets.stream())
+                .anyMatch(rollingStock -> rollingStock.getIdentifier().equals(id))) {
+            throw new InvalidInputException("engine identifier already used");
         }
         engines.add(newEngine);
     }
@@ -212,15 +203,9 @@ public class ModelRailwaySimulation {
      */
     public void createTrainSet(final TrainSet newTrainSet) throws InvalidInputException {
         String id = newTrainSet.getIdentifier();
-        for (Engine engine : engines) {
-            if (engine.getIdentifier().equals(id)) {
-                throw new InvalidInputException("train set identifier already used by engine");
-            }
-        }
-        for (TrainSet trainSet : trainSets) {
-            if (trainSet.getIdentifier().equals(id)) {
-                throw new InvalidInputException("train set identifier already used by train set");
-            }
+        if (Stream.concat(engines.stream(), trainSets.stream())
+                .anyMatch(rollingStock -> rollingStock.getIdentifier().equals(id))) {
+            throw new InvalidInputException("train set identifier already used");
         }
         trainSets.add(newTrainSet);
     }
@@ -288,10 +273,8 @@ public class ModelRailwaySimulation {
         if (rollingStock == null) {
             throw new InvalidInputException("rolling stock not found");
         }
-        for (Train train : trains.values()) {
-            if (train.contains(rollingStock)) {
-                throw new InvalidInputException("rolling stock already used");
-            }
+        if (trains.values().stream().anyMatch(train -> train.contains(rollingStock))) {
+            throw new InvalidInputException("rolling stock already used");
         }
         Train train = trains.get(trainId);
         if (train != null && train.isPlaced()) {
@@ -299,14 +282,14 @@ public class ModelRailwaySimulation {
         }
         if (train != null) {
             train.add(rollingStock);
-            return;
+        } else {
+            int correctId = getNextTrainIdentifier();
+            if (trainId != correctId) {
+                throw new InvalidInputException("new train identifier must be next free identifier");
+            }
+            Train newTrain = new Train(trainId, rollingStock);
+            trains.put(trainId, newTrain);
         }
-        int correctId = getNextTrainIdentifier();
-        if (trainId != correctId) {
-            throw new InvalidInputException("new train identifier must be next free identifier");
-        }
-        Train newTrain = new Train(trainId, rollingStock);
-        trains.put(trainId, newTrain);
     }
 
     /**
@@ -569,18 +552,17 @@ public class ModelRailwaySimulation {
             return;
         }
 
-        List<Set<Train>> collisions = new ArrayList<>();
-        // first, handle positive speed (forward)
-        for (int i = 0; i < speed; i++) {
-            List<Set<Train>> newCollisions = getCollisionsOfOneStep();
-            collisions.addAll(newCollisions);
+        List<Set<Train>> collisions;
+        if (speed >= 0) {
+            collisions = IntStream.range(0, speed)
+                    .mapToObj(step -> getCollisionsOfOneStep())
+                    .flatMap(List::stream).collect(Collectors.toList());
+        } else {
+            collisions = IntStream.range(0, -speed)
+                    .mapToObj(step -> getCollisionsOfOneReverseStep())
+                    .flatMap(List::stream).collect(Collectors.toList());
         }
-        // then, handle negative speed (backward)
-        for (int i = 0; i > speed; i--) {
-            List<Set<Train>> newCollisions = getCollisionsOfOneReverseStep();
-            collisions.addAll(newCollisions);
-        }
-        // TODO (only one of the loops above actually runs)
+
         for (int id : trains.keySet().stream().sorted().collect(Collectors.toList())) {
             Train train = trains.get(id);
             Set<Train> collisionSet = collisions.stream()
diff --git a/src/edu/kit/informatik/model/Train.java b/src/edu/kit/informatik/model/Train.java
index 8f9d44b..680684b 100644
--- a/src/edu/kit/informatik/model/Train.java
+++ b/src/edu/kit/informatik/model/Train.java
@@ -129,7 +129,7 @@ public final class Train {
      * @return whether this train is on that rail
      */
     public boolean isOnRail(final int id) {
-        return occupiedRails != null && occupiedRails.stream().anyMatch(rail -> rail.getIdentifier() == id);
+        return isPlaced() && occupiedRails.stream().anyMatch(rail -> rail.getIdentifier() == id);
     }
 
     /**