Index: vehicle.c
===================================================================
--- vehicle.c	(revision 5082)
+++ vehicle.c	(working copy)
@@ -1748,6 +1748,7 @@
 	}
 
 	for (;;) {
+		uint length = 0;
 		cost = 0;
 		w = v;
 		do {
@@ -1756,21 +1757,29 @@
 				continue;
 			}
 
-			// check if the vehicle should be replaced
-			if (!p->engine_renew ||
-					w->age - w->max_age < (p->engine_renew_months * 30) || // replace if engine is too old
-					w->max_age == 0) { // rail cars got a max age of 0
-				if (!EngineHasReplacementForPlayer(p, w->engine_type)) // updates to a new model
-					continue;
-			}
-
-			if (w->type == VEH_Train && IsTrainWagon(w)) {
-				EngineID e = EngineReplacementForPlayer(p, w->engine_type);
+			if (!EngineHasReplacementForPlayer(p, w->engine_type)) {
+				if (w->type == VEH_Train) {
+					length += RailVehInfo(w->engine_type)->flags & RVI_MULTIHEAD ? 2 : 1;
+				}
 
-				if (w->cargo_type != RailVehInfo(e)->cargo_type && !CanRefitTo(e, w->cargo_type)) {
-					// we can't replace this wagon since the cargo type is incorrent, and we can't refit it
+				/* This engine/wagon has no replacement model, so check if it
+				 * is to be replaced due to age. */
+				if (!p->engine_renew || w->max_age == 0 ||
+						w->age - w->max_age < (p->engine_renew_months * 30)) {
 					continue;
 				}
+			} else {
+				if (w->type == VEH_Train) {
+					EngineID e = EngineReplacementForPlayer(p, w->engine_type);
+
+					/* Add number of engines & wagons to the total length */
+					length += RailVehInfo(e)->flags & RVI_MULTIHEAD ? 2 : 1;
+
+					if (IsTrainWagon(w) && w->cargo_type != RailVehInfo(e)->cargo_type && !CanRefitTo(e, w->cargo_type)) {
+						// we can't replace this wagon since the cargo type is incorrent, and we can't refit it
+						continue;
+					}
+				}
 			}
 
 			/* Now replace the vehicle */
@@ -1790,6 +1799,20 @@
 			}
 		} while (w->type == VEH_Train && (w = GetNextVehicle(w)) != NULL);
 
+		/* Check if the new length would exceed the maximum number of wagons */
+		if (v->type == VEH_Train && length > (_patches.mammoth_trains ? 100 : 9)) {
+			/* Send a news message to the train's owner */
+			if (_local_player == v->owner) {
+				SetDParam(0, v->unitnumber);
+				AddNewsItem(STR_TRAIN_TOO_LONG_AFTER_REPLACEMENT, NEWS_FLAGS(NM_SMALL, NF_VIEWPORT|NF_VEHICLE, NT_ADVICE, 0), v->index, 0);
+			}
+
+			/* Send the train on its way */
+			if (stopped) v->vehstatus &= ~VS_STOPPED;
+			_current_player = OWNER_NONE;
+			return;
+		}
+
 		if (p->money64 < (int32)(cost + p->engine_renew_money) || cost == 0) {
 			if (p->money64 < (int32)(cost + p->engine_renew_money) && ( _local_player == v->owner ) && cost != 0) {
 				StringID message;