diff --git a/src/track_func.h b/src/track_func.h
--- a/src/track_func.h
+++ b/src/track_func.h
@@ -14,6 +14,7 @@
 
 #include "core/bitmath_func.hpp"
 #include "track_type.h"
+#include "direction_func.h"
 #include "slope_func.h"
 
 /**
@@ -28,6 +29,42 @@
 #define FOR_EACH_SET_TRACK(var, track_bits) FOR_EACH_SET_BIT_EX(Track, var, TrackBits, track_bits)
 
 /**
+ * Checks if a Track is valid.
+ *
+ * @param track The value to check
+ * @return true if the given value is a valid track.
+ * @note Use this in an assert()
+ */
+static inline bool IsValidTrack(Track track)
+{
+	return track < TRACK_END;
+}
+
+/**
+ * Checks if a Trackdir is valid for road vehicles.
+ *
+ * @param trackdir The value to check
+ * @return true if the given value is a valid Trackdir
+ * @note Use this in an assert()
+ */
+static inline bool IsValidTrackdirForRoadVehicle(Trackdir trackdir)
+{
+	return trackdir < TRACKDIR_END;
+}
+
+/**
+ * Checks if a Trackdir is valid for non-road vehicles.
+ *
+ * @param trackdir The value to check
+ * @return true if the given value is a valid Trackdir
+ * @note Use this in an assert()
+ */
+static inline bool IsValidTrackdir(Trackdir trackdir)
+{
+	return (1 << trackdir & TRACKDIR_BIT_MASK) != 0;
+}
+
+/**
  * Convert an Axis to the corresponding Track
  * AXIS_X -> TRACK_X
  * AXIS_Y -> TRACK_Y
@@ -38,6 +75,7 @@
  */
 static inline Track AxisToTrack(Axis a)
 {
+	assert(IsValidAxis(a));
 	return (Track)a;
 }
 
@@ -48,6 +86,7 @@ static inline Track AxisToTrack(Axis a)
  */
 static inline TrackBits TrackToTrackBits(Track track)
 {
+	assert(IsValidTrack(track));
 	return (TrackBits)(1 << track);
 }
 
@@ -74,8 +113,6 @@ static inline TrackBits CornerToTrackBit
 	return _corner_to_trackbits[corner];
 }
 
-
-
 /**
  * Maps a Trackdir to the corresponding TrackdirBits value
  * @param trackdir the track direction to convert
@@ -83,6 +120,7 @@ static inline TrackBits CornerToTrackBit
  */
 static inline TrackdirBits TrackdirToTrackdirBits(Trackdir trackdir)
 {
+	assert(IsValidTrackdir(trackdir));
 	return (TrackdirBits)(1 << trackdir);
 }
 
@@ -103,6 +141,7 @@ static inline TrackdirBits TrackdirToTra
 static inline Track RemoveFirstTrack(TrackBits *tracks)
 {
 	if (*tracks != TRACK_BIT_NONE && *tracks != INVALID_TRACK_BIT) {
+		assert((*tracks & ~TRACK_BIT_MASK) == TRACK_BIT_NONE);
 		Track first = (Track)FIND_FIRST_BIT(*tracks);
 		ClrBit(*tracks, first);
 		return first;
@@ -127,6 +166,7 @@ static inline Track RemoveFirstTrack(Tra
 static inline Trackdir RemoveFirstTrackdir(TrackdirBits *trackdirs)
 {
 	if (*trackdirs != TRACKDIR_BIT_NONE && *trackdirs != INVALID_TRACKDIR_BIT) {
+		assert((*trackdirs & ~TRACKDIR_BIT_MASK) == TRACKDIR_BIT_NONE);
 		Trackdir first = (Trackdir)FindFirstBit2x64(*trackdirs);
 		ClrBit(*trackdirs, first);
 		return first;
@@ -184,31 +224,6 @@ static inline Trackdir FindFirstTrackdir
 	return (trackdirs != TRACKDIR_BIT_NONE) ? (Trackdir)FindFirstBit2x64(trackdirs) : INVALID_TRACKDIR;
 }
 
-/**
- * Checks if a Track is valid.
- *
- * @param track The value to check
- * @return true if the given value is a valid track.
- * @note Use this in an assert()
- */
-static inline bool IsValidTrack(Track track)
-{
-	return track < TRACK_END;
-}
-
-/**
- * Checks if a Trackdir is valid.
- *
- * @param trackdir The value to check
- * @return true if the given value is a valid Trackdir
- * @note Use this in an assert()
- */
-static inline bool IsValidTrackdir(Trackdir trackdir)
-{
-	return (TrackdirToTrackdirBits(trackdir) & TRACKDIR_BIT_MASK) != 0;
-}
-
-
 /*
  * Functions describing logical relations between Tracks, TrackBits, Trackdirs
  * TrackdirBits, Direction and DiagDirections.
@@ -225,7 +240,7 @@ static inline bool IsValidTrackdir(Track
  */
 static inline Track TrackToOppositeTrack(Track t)
 {
-	assert(t != INVALID_TRACK);
+	assert(IsValidTrack(t));
 	return (Track)(t ^ 1);
 }
 
@@ -241,7 +256,7 @@ static inline Track TrackToOppositeTrack
  */
 static inline Trackdir ReverseTrackdir(Trackdir trackdir)
 {
-	assert(trackdir != INVALID_TRACKDIR);
+	assert(IsValidTrackdirForRoadVehicle(trackdir));
 	return (Trackdir)(trackdir ^ 8);
 }
 
@@ -256,6 +271,7 @@ static inline Trackdir ReverseTrackdir(T
  */
 static inline Track TrackdirToTrack(Trackdir trackdir)
 {
+	assert(IsValidTrackdir(trackdir));
 	return (Track)(trackdir & 0x7);
 }
 
@@ -272,6 +288,7 @@ static inline Track TrackdirToTrack(Trac
  */
 static inline Trackdir TrackToTrackdir(Track track)
 {
+	assert(IsValidTrack(track));
 	return (Trackdir)track;
 }
 
@@ -373,6 +390,7 @@ static inline TrackStatus CombineTrackSt
  */
 static inline Trackdir NextTrackdir(Trackdir trackdir)
 {
+	assert(IsValidTrackdir(trackdir));
 	extern const Trackdir _next_trackdir[TRACKDIR_END];
 	return _next_trackdir[trackdir];
 }
@@ -389,6 +407,7 @@ static inline Trackdir NextTrackdir(Trac
  */
 static inline TrackBits TrackCrossesTracks(Track track)
 {
+	assert(IsValidTrack(track));
 	extern const TrackBits _track_crosses_tracks[TRACK_END];
 	return _track_crosses_tracks[track];
 }
@@ -407,6 +426,7 @@ static inline TrackBits TrackCrossesTrac
  */
 static inline DiagDirection TrackdirToExitdir(Trackdir trackdir)
 {
+	assert(IsValidTrackdirForRoadVehicle(trackdir));
 	extern const DiagDirection _trackdir_to_exitdir[TRACKDIR_END];
 	return _trackdir_to_exitdir[trackdir];
 }
@@ -428,6 +448,8 @@ static inline DiagDirection TrackdirToEx
  */
 static inline Trackdir TrackExitdirToTrackdir(Track track, DiagDirection diagdir)
 {
+	assert(IsValidTrack(track));
+	assert(IsValidDiagDirection(diagdir));
 	extern const Trackdir _track_exitdir_to_trackdir[TRACK_END][DIAGDIR_END];
 	return _track_exitdir_to_trackdir[track][diagdir];
 }
@@ -451,6 +473,8 @@ static inline Trackdir TrackExitdirToTra
  */
 static inline Trackdir TrackEnterdirToTrackdir(Track track, DiagDirection diagdir)
 {
+	assert(IsValidTrack(track));
+	assert(IsValidDiagDirection(diagdir));
 	extern const Trackdir _track_enterdir_to_trackdir[TRACK_END][DIAGDIR_END];
 	return _track_enterdir_to_trackdir[track][diagdir];
 }
@@ -461,6 +485,8 @@ static inline Trackdir TrackEnterdirToTr
  */
 static inline Trackdir TrackDirectionToTrackdir(Track track, Direction dir)
 {
+	assert(IsValidTrack(track));
+	assert(IsValidDirection(dir));
 	extern const Trackdir _track_direction_to_trackdir[TRACK_END][DIR_END];
 	return _track_direction_to_trackdir[track][dir];
 }
@@ -473,6 +499,7 @@ static inline Trackdir TrackDirectionToT
  */
 static inline Track DiagDirToDiagTrack(DiagDirection diagdir)
 {
+	assert(IsValidDiagDirection(diagdir));
 	return (Track)(diagdir & 1);
 }
 
@@ -484,6 +511,7 @@ static inline Track DiagDirToDiagTrack(D
  */
 static inline TrackBits DiagDirToDiagTrackBits(DiagDirection diagdir)
 {
+	assert(IsValidDiagDirection(diagdir));
 	return TrackToTrackBits(DiagDirToDiagTrack(diagdir));
 }
 
@@ -496,6 +524,7 @@ static inline TrackBits DiagDirToDiagTra
  */
 static inline Trackdir DiagDirToDiagTrackdir(DiagDirection diagdir)
 {
+	assert(IsValidDiagDirection(diagdir));
 	extern const Trackdir _dir_to_diag_trackdir[DIAGDIR_END];
 	return _dir_to_diag_trackdir[diagdir];
 }
@@ -513,6 +542,7 @@ static inline Trackdir DiagDirToDiagTrac
  */
 static inline TrackdirBits DiagdirReachesTrackdirs(DiagDirection diagdir)
 {
+	assert(IsValidDiagDirection(diagdir));
 	extern const TrackdirBits _exitdir_reaches_trackdirs[DIAGDIR_END];
 	return _exitdir_reaches_trackdirs[diagdir];
 }
@@ -541,6 +571,7 @@ static inline TrackBits DiagdirReachesTr
  */
 static inline TrackdirBits TrackdirReachesTrackdirs(Trackdir trackdir)
 {
+	assert(IsValidTrackdir(trackdir));
 	extern const TrackdirBits _exitdir_reaches_trackdirs[DIAGDIR_END];
 	return _exitdir_reaches_trackdirs[TrackdirToExitdir(trackdir)];
 }
@@ -562,6 +593,7 @@ static inline TrackdirBits TrackdirReach
  */
 static inline TrackdirBits TrackdirCrossesTrackdirs(Trackdir trackdir)
 {
+	assert(IsValidTrackdirForRoadVehicle(trackdir));
 	extern const TrackdirBits _track_crosses_trackdirs[TRACKDIR_END];
 	return _track_crosses_trackdirs[TrackdirToTrack(trackdir)];
 }
@@ -574,6 +606,7 @@ static inline TrackdirBits TrackdirCross
  */
 static inline bool IsDiagonalTrack(Track track)
 {
+	assert(IsValidTrack(track));
 	return (track == TRACK_X) || (track == TRACK_Y);
 }
 
@@ -585,6 +618,7 @@ static inline bool IsDiagonalTrack(Track
  */
 static inline bool IsDiagonalTrackdir(Trackdir trackdir)
 {
+	assert(IsValidTrackdir(trackdir));
 	return IsDiagonalTrack(TrackdirToTrack(trackdir));
 }
 
@@ -626,6 +660,7 @@ static inline bool TrackOverlapsTracks(T
  */
 static inline bool IsReversingRoadTrackdir(Trackdir dir)
 {
+	assert(IsValidTrackdirForRoadVehicle(dir));
 	return (dir & 0x07) >= 6;
 }
 
@@ -636,6 +671,7 @@ static inline bool IsReversingRoadTrackd
  */
 static inline bool IsStraightRoadTrackdir(Trackdir dir)
 {
+	assert(IsValidTrackdirForRoadVehicle(dir));
 	return (dir & 0x06) == 0;
 }
 
@@ -651,6 +687,7 @@ static inline bool IsStraightRoadTrackdi
  */
 static inline bool IsUphillTrackdir(Slope slope, Trackdir dir)
 {
+	assert(IsValidTrackdirForRoadVehicle(dir));
 	extern const TrackdirBits _uphill_trackdirs[];
 	return HasBit(_uphill_trackdirs[RemoveHalftileSlope(slope)], dir);
 }