Try splitting into regional fields

data/fields/crossing/whistle_optional.json

@@ -2,13 +2,18 @@
     "key": "crossing:whistle",
     "type": "combo",
     "label": "Train Horn",
+    "placeholder": "Only as Needed",
     "strings": {
         "options": {
             "yes": "Mandatory",
-            "optional": "Only as Needed",
             "wayside": "Wayside/Stationary Horn"
         }
     },
     "autoSuggestions": false,
-    "customValues": false
+    "customValues": false,
+    "locationSet": {
+        "include": [
+            "150"
+        ]
+    }
 }

data/fields/crossing/whistle_yes.json

@@ -0,0 +1,20 @@
+{
+    "key": "crossing:whistle",
+    "type": "combo",
+    "label": "Train Horn",
+    "placeholder": "Mandatory",
+    "strings": {
+        "options": {
+            "optional": "Only as Needed",
+            "wayside": "Wayside/Stationary Horn"
+        }
+    },
+    "autoSuggestions": false,
+    "customValues": false,
+    "locationSet": {
+        "include": [
+            "ca",
+            "us"
+        ]
+    }
+}

data/presets/railway/crossing.json

@@ -8,7 +8,8 @@
     ],
     "moreFields": [
         "crossing/markings",
-        "crossing/whistle"
+        "crossing/whistle_optional",
+        "crossing/whistle_yes"
     ],
     "geometry": [
         "vertex"

data/presets/railway/level_crossing.json

@@ -6,7 +6,8 @@
         "crossing/light"
     ],
     "moreFields": [
-        "crossing/whistle"
+        "crossing/whistle_optional",
+        "crossing/whistle_yes"
     ],
     "geometry": [
         "vertex"