diff --git a/custom_components/mbtalive/config_flow.py b/custom_components/mbtalive/config_flow.py
index 6c464f0..53fa00a 100644
--- a/custom_components/mbtalive/config_flow.py
+++ b/custom_components/mbtalive/config_flow.py
@@ -1,4 +1,5 @@
 import logging
+import re
 import voluptuous as vol
 from typing import Any, Dict, Optional
 
@@ -14,6 +15,16 @@
 
 _LOGGER = logging.getLogger(__name__)
 
+# --- Custom validator for train number ---
+def validate_train_number(value: str | None) -> str | None:
+    """Validate that train number is 2–4 alphanumeric characters or empty."""
+    if value in ("", None):
+        return None  # normalize empty
+    value = str(value).strip().upper()
+    if not re.fullmatch(r"[A-Z0-9]{2,4}", value):
+        raise vol.Invalid("Train number must be 2–4 letters/numbers or empty")
+    return value
+
 
 def get_user_schema(default_api_key: str = "") -> vol.Schema:
     return vol.Schema({
@@ -21,10 +32,7 @@ def get_user_schema(default_api_key: str = "") -> vol.Schema:
         vol.Required("arrive_at", default=""): str,
         vol.Required("api_key", default=default_api_key): vol.All(str, vol.Length(min=32, max=32)),
         vol.Optional("max_trips", default=2): int,
-        vol.Optional("train", default=""): vol.Any(
-            vol.All(str, vol.Length(min=3, max=3)),
-            ""
-        )
+        vol.Optional("train", default=""): validate_train_number,
     })
 
 
@@ -45,6 +53,10 @@ async def async_step_user(
             arrive_at = user_input.get("arrive_at")
             api_key = user_input.get("api_key", "").strip()
             train = user_input.get("train", "")
+            # Normalize empty train to None
+            if train in ("", None):
+                train = None
+                user_input["train"] = None
 
             if not api_key:
                 errors["api_key"] = "required"