diff --git a/rail.h b/rail.h
--- a/rail.h
+++ b/rail.h
@@ -42,7 +42,7 @@ typedef enum RailTileSubtypes {
 } RailTileSubtype;
 
 typedef enum SignalTypes {
-	/* Stored in m4[0..1] for MP_RAILWAY */
+	/* Stored in m4[0..2] for MP_RAILWAY */
 	SIGTYPE_NORMAL  = 0,        // normal signal
 	SIGTYPE_ENTRY   = 1,        // presignal block entry
 	SIGTYPE_EXIT    = 2,        // presignal block exit
diff --git a/rail_cmd.c b/rail_cmd.c
--- a/rail_cmd.c
+++ b/rail_cmd.c
@@ -757,7 +757,7 @@ int32 CmdBuildSingleSignal(int x, int y,
 			_m[tile].m5 |= RAIL_TYPE_SIGNALS; // change into signals
 			_m[tile].m2 |= 0xF0;              // all signals are on
 			_m[tile].m3 &= ~0xF0;          // no signals built by default
-			_m[tile].m4 = semaphore ? 0x08 : 0;
+			_m[tile].m4 = semaphore ? SIG_SEMAPHORE_MASK : 0;
 		}
 
 		if (p2 == 0) {
@@ -767,7 +767,7 @@ int32 CmdBuildSingleSignal(int x, int y,
 			} else {
 				if (pre_signal) {
 					// cycle between normal -> pre -> exit -> combo -> pbs ->...
-					byte type = ((GetSignalType(tile, track) + 1) % 5);
+					byte type = (GetSignalType(tile, track) + 1) % SIGTYPE_END;
 					SB(_m[tile].m4, 0, 3, type);
 				} else {
 					// cycle between two-way -> one-way -> one-way -> ...