diff --git a/macros.h b/macros.h
--- a/macros.h
+++ b/macros.h
@@ -130,7 +130,14 @@ extern uint SafeTileAdd(uint x, int add,
 #define SPRITE_PALETTE(x) ((x) + 0x8000)
 
 extern const byte _ffb_64[128];
+/* Returns the position of the first bit that is not zero, counted from the
+ * left. Ie, 10110100 returns 2, 00000001 returns 0, etc. When x == 0 returns
+ * 0.
+ */
 #define FIND_FIRST_BIT(x) _ffb_64[(x)]
+/* Returns x with the first bit that is not zero, counted from the left, set
+ * to zero. So, 10110100 returns 10110000, 00000001 returns 00000000, etc.
+ */
 #define KILL_FIRST_BIT(x) _ffb_64[(x)+64]
 
 static INLINE int FindFirstBit2x64(int value)
@@ -212,4 +219,6 @@ static INLINE void WRITE_LE_UINT16(const
 	((byte*)b)[1] = (byte)(x >> 8);
 }
 
+#define MAX_DETOUR 6
+
 #endif /* MACROS_H */
diff --git a/pathfind.c b/pathfind.c
--- a/pathfind.c
+++ b/pathfind.c
@@ -187,6 +187,9 @@ continue_here:;
 
 static const int8 _get_tunlen_inc[5] = { -16, 0, 16, 0, -16 };
 
+/* Returns the end tile and the length of a tunnel. The length does not
+ * include the starting tile (entry), it does include the end tile (exit).
+ */
 FindLengthOfTunnelResult FindLengthOfTunnel(uint tile, int direction, byte type)
 {
 	FindLengthOfTunnelResult flotr;
diff --git a/rail_cmd.c b/rail_cmd.c
--- a/rail_cmd.c
+++ b/rail_cmd.c
@@ -8,7 +8,7 @@
 
 void ShowTrainDepotWindow(uint tile);
 
-enum {
+enum { /* These values are bitmasks for the map5 byte */
 	RAIL_TYPE_NORMAL = 0,
 	RAIL_TYPE_SIGNALS = 0x40,
 	RAIL_TYPE_SPECIAL = 0x80, // If this bit is set, then it's not a regular track.
@@ -40,6 +40,9 @@ enum {
  * 01 abcdef  => Rail with signals
  * 10 ??????  => Unused
  * 11 ????dd  => Depot
+ *
+ * abcdef is a bitmask, which contains ones for all present tracks. Below the
+ * value for each track is given. 
  */
 
 /*         4
@@ -77,8 +80,12 @@ enum RailMap2Lower4 {
 };
 
 
-/* MAP2 byte:    abcd???? => Signal On?
- * MAP3LO byte:  abcd???? => Signal Exists?
+/* MAP2 byte:    abcd???? => Signal On? Same coding as map3lo
+ * MAP3LO byte:  abcd???? => Signal Exists? 
+ *				 a and b are for diagonals, upper and left,
+ *				 one for each direction. (ie a == NE->SW, b ==
+ *				 SW->NE, or v.v., I don't know. b and c are
+ *				 similar for lower and right.
  * MAP2 byte:    ????abcd => Type of ground.
  * MAP3LO byte:  ????abcd => Type of rail.
  * MAP5:         00abcdef => rail
@@ -1796,6 +1803,7 @@ static uint32 GetTileTrackStatus_Track(u
 		ret = (m5 | (m5 << 8)) & 0x3F3F;
 		if (!(m5 & RAIL_TYPE_SIGNALS)) {
 			if ( (ret & 0xFF) == 3)
+			/* Diagonal crossing? */
 				ret |= 0x40;
 		} else {
 			/* has_signals */
@@ -1805,6 +1813,10 @@ static uint32 GetTileTrackStatus_Track(u
 
 			b &= a;
 
+			/* When signals are not present (in neither
+			 * direction), we pretend them to be green. (So if
+			 * signals are only one way, the other way will
+			 * implicitely become `red' */
 			if ((a & 0xC0) == 0) { b |= 0xC0; }
 			if ((a & 0x30) == 0) { b |= 0x30; }
 
diff --git a/road_cmd.c b/road_cmd.c
--- a/road_cmd.c
+++ b/road_cmd.c
@@ -7,6 +7,10 @@
 #include "town.h"
 #include "gfx.h"
 
+/* When true, GetTrackStatus for roads will treat roads under reconstruction
+ * as normal roads instead of impassable. This is used when detecting whether
+ * a road can be removed. This is of course ugly, but I don't know a better
+ * solution just like that... */
 static bool _road_special_gettrackstatus;
 
 void RoadVehEnterDepot(Vehicle *v);
diff --git a/station_cmd.c b/station_cmd.c
--- a/station_cmd.c
+++ b/station_cmd.c
@@ -2232,8 +2232,8 @@ uint MoveGoodsToStation(uint tile, int w
 							st->blocked_months == 0 &&
 							st->goods[type].rating != 0 && 
 							(!_patches.selectgoods || st->goods[type].last_speed) && // if last_speed is 0, no vehicle has been there.
-							((st->facilities & (byte)~FACIL_BUS_STOP)!=0 || type==CT_PASSENGERS) &&
-							((st->facilities & (byte)~FACIL_TRUCK_STOP)!=0 || type!=CT_PASSENGERS)) {
+							((st->facilities & (byte)~FACIL_BUS_STOP)!=0 || type==CT_PASSENGERS) && // if we have other fac. than a bus stop, or the cargo is passengers
+							((st->facilities & (byte)~FACIL_TRUCK_STOP)!=0 || type!=CT_PASSENGERS)) { // if we have other fac. than a cargo bay or the cargo is not passengers
 						
 						around[i] = st_index;
 						around_ptr[i] = st;
diff --git a/train_cmd.c b/train_cmd.c
--- a/train_cmd.c
+++ b/train_cmd.c
@@ -1921,6 +1921,12 @@ static int CountPassengersInTrain(Vehicl
 	return num;
 }
 
+/*
+ * Checks whether the specified tried has a collision with another vehicle. If
+ * so, destroys this vehicle, and the other vehicle if its subtype is 0 (?).
+ * Reports the incident in a flashy news item, modifies station ratings and
+ * plays a sound.
+ */
 static void CheckTrainCollision(Vehicle *v)
 {
 	TrainCollideChecker tcc;
@@ -1991,18 +1997,20 @@ static void TrainController(Vehicle *v)
 	byte chosen_track;
 	byte old_z;
 
+	/* For every vehicle after and including the given vehicle */
 	for(;;) {
 		BeginVehicleMove(v);
 		
 		if (v->u.rail.track != 0x40) {
+			/* Not inside tunnel */
 			if (GetNewVehiclePos(v, &gp)) {
-				/* Statying in the old tile */
+				/* Staying in the old tile */
 				if (v->u.rail.track == 0x80) {
 					/* inside depot */
 					gp.x = v->x_pos;
 					gp.y = v->y_pos;
 				} else {
-					/* isnot inside depot */
+					/* is not inside depot */
 					r = VehicleEnterTile(v, gp.new_tile, gp.x, gp.y);
 					if (r & 0x8)
 						goto invalid_rail;
@@ -2079,6 +2087,8 @@ static void TrainController(Vehicle *v)
 				if (v->subtype == 0)
 					TrainMovedChangeSignals(gp.new_tile, dir>>1);
 
+				/* Signals can only change when the first
+				 * (above) or the last vehicle moves. */
 				if (v->next == NULL)
 					TrainMovedChangeSignals(gp.old_tile, (dir>>1) ^ 2);
 				
@@ -2117,24 +2127,27 @@ common:;
 		old_z = AfterSetTrainPos(v);
 		
 		if (prev == NULL) {
+			/* This is the first vehicle in the train */
 			AffectSpeedByZChange(v, old_z);
 			CheckTrainCollision(v);
 		}
 
+next_vehicle:;
 		/* continue with next vehicle */
-next_vehicle:;
 		prev = v;
 		if ((v=v->next) == NULL)
 			return;
 	}
 
 invalid_rail:
+	/* We've reached end of line?? */
 	if (prev != NULL) {
 		error("!Disconnecting train");
 	}
 	goto reverse_train_direction;
 
 red_light: {
+	/* We're in front of a red signal ?? */
 		/* find the first set bit in ts. need to do it in 2 steps, since
 		 * FIND_FIRST_BIT only handles 6 bits at a time. */
 		i = FindFirstBit2x64(ts);
@@ -2307,14 +2320,17 @@ static void TrainCheckIfLineEnds(Vehicle
 	if (IS_TILETYPE(tile, MP_RAILWAY) && (_map5[tile] & 0xFC) == 0xC0)
 		return;
 
-	// determine the tracks on the next tile.
+	/* Determine the non-diagonal direction in which we will exit this tile */
 	t = v->direction >> 1;
 	if (!(v->direction & 1) && v->u.rail.track != _state_dir_table[t]) {
 		t = (t - 1) & 3;
 	}
+	/* Calculate next tile */
 	tile += _tileoffs_by_dir[t];
+	// determine the track status on the next tile.
 	ts = GetTileTrackStatus(tile, 0) & _reachable_tracks[t];
 	
+	/* Calc position within the current tile ?? */
 	x = v->x_pos & 0xF;
 	y = v->y_pos & 0xF;
 	
diff --git a/train_gui.c b/train_gui.c
--- a/train_gui.c
+++ b/train_gui.c
@@ -316,8 +316,10 @@ static void DrawTrainDepotWindow(Window 
 				--num < 0 && num >= -6) {
 
 			DrawTrainImage(v, x+21, y, 10, w->hscroll.pos, WP(w,traindepot_d).sel);
+			/* Draw the train number */
 			SET_DPARAM16(0, v->unitnumber);
 			DrawString(x, y, (v->max_age - 366 < v->age) ? STR_00E3 : STR_00E2, 0);
+			/* Draw the pretty flag */
 			DrawSprite(v->vehstatus&VS_STOPPED ? 0xC12 : 0xC13, x+15, y);
 
 			y += 14;
diff --git a/ttd.h b/ttd.h
--- a/ttd.h
+++ b/ttd.h
@@ -236,6 +236,10 @@ typedef void ClickTileProc(uint tile);
 typedef void AnimateTileProc(uint tile);
 typedef void TileLoopProc(uint tile);
 typedef void ChangeTileOwnerProc(uint tile, byte old_player, byte new_player);
+/* Return value has bit 0x2 set, when the vehicle enters a station. Then,
+ * result << 8 contains the id of the station entered. If the return value has
+ * bit 0x8 set, the vehicle could not and did not enter the tile. Are there
+ * other bits that can be set? */
 typedef uint32 VehicleEnterTileProc(Vehicle *v, uint tile, int x, int y);
 typedef void VehicleLeaveTileProc(Vehicle *v, uint tile, int x, int y);
 
diff --git a/tunnelbridge_cmd.c b/tunnelbridge_cmd.c
--- a/tunnelbridge_cmd.c
+++ b/tunnelbridge_cmd.c
@@ -1388,6 +1388,7 @@ static uint32 VehicleEnter_TunnelBridge(
 			}
 
 			if (dir == (vdir^2) && fc == _tunnel_fractcoord_3[dir] && z == 0) {
+				/* We're at the tunnel exit ?? */
 				v->tile = tile;
 				v->u.rail.track = _exit_tunnel_track[dir];
 				v->vehstatus &= ~VS_HIDDEN;
@@ -1413,6 +1414,7 @@ static uint32 VehicleEnter_TunnelBridge(
 			}
 
 			if (dir == (vdir^2) && (
+				/* We're at the tunnel exit ?? */
 					fc == _tunnel_fractcoord_6[dir] ||
 					fc == _tunnel_fractcoord_7[dir]) &&
 					z == 0) {
diff --git a/vehicle.c b/vehicle.c
--- a/vehicle.c
+++ b/vehicle.c
@@ -1458,11 +1458,18 @@ byte GetDirectionTowards(Vehicle *v, int
 	return (dir+((dirdiff&7)<5?1:-1)) & 7;
 }
 
+/* Return value has bit 0x2 set, when the vehicle enters a station. Then,
+ * result << 8 contains the id of the station entered. If the return value has
+ * bit 0x8 set, the vehicle could not and did not enter the tile. Are there
+ * other bits that can be set? */
 uint32 VehicleEnterTile(Vehicle *v, uint tile, int x, int y)
 {	
 	uint old_tile = v->tile;
 	uint32 result = _tile_type_procs[GET_TILETYPE(tile)]->vehicle_enter_tile_proc(v, tile, x, y);
 	
+	/* When vehicle_enter_tile_proc returns 8, that apparently means that
+	 * we cannot enter the tile at all. In that case, don't call
+	 * leave_tile. */
 	if (!(result & 8) && old_tile != tile) {
 		VehicleLeaveTileProc *proc = _tile_type_procs[GET_TILETYPE(old_tile)]->vehicle_leave_tile_proc;
 		if (proc != NULL)
diff --git a/vehicle.h b/vehicle.h
--- a/vehicle.h
+++ b/vehicle.h
@@ -85,7 +85,7 @@ struct WorldSprite {
 
 struct Vehicle {
 	byte type;				// type, ie roadven,train,ship,aircraft,special
-	byte subtype;			// subtype
+	byte subtype;			// subtype (for trains, 0 == loco, 4 wagon ??)
 
 	uint16 index;			// NOSAVE: Index in vehicle array
 	uint16 next_in_chain_old; // Next vehicle index for chained vehicles