diff --git a/.forgejo/workflows/test.yml b/.forgejo/workflows/test.yml
new file mode 100644
index 0000000..47e398d
--- /dev/null
+++ b/.forgejo/workflows/test.yml
@@ -0,0 +1,16 @@
+name: GTFS Route Modification
+
+on: [push]
+
+jobs:
+  modify_routes:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Run GTFS Route Modifier
+        uses: gtfs-actions/route-type-modifier@v1
+        with:
+          gtfs_file: 'path/to/your/gtfs.zip'
+          routes: '1:3,2:5'
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diff --git a/Dockerfile b/Dockerfile
new file mode 100644
index 0000000..fd758cf
--- /dev/null
+++ b/Dockerfile
@@ -0,0 +1,11 @@
+FROM python:3.12-slim
+
+WORKDIR /app
+
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
+COPY modify_gtfs.py .
+COPY action.yml .
+
+ENTRYPOINT ["python", "modify_gtfs.py"]
diff --git a/README.md b/README.md
index 02bf09a..1159d0d 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,20 @@
-# route-type-modifier
+# GTFS Route Type Modifier
+### Usage
+```yaml
+name: GTFS Route Type Modification
 
+on: [push]
+
+jobs:
+  modify_routes:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Run GTFS Route Modifier
+        uses: gtfs-actions/route-type-modifier@v1
+        with:
+          gtfs_file: 'path/to/your/gtfs.zip'
+          routes: '1:3,2:5'
+```
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diff --git a/action.yml b/action.yml
new file mode 100644
index 0000000..a8804d0
--- /dev/null
+++ b/action.yml
@@ -0,0 +1,18 @@
+name: 'GTFS Route Type Modifier'
+description: 'Change route type in a GTFS feed.'
+
+inputs:
+  gtfs_file:
+    description: 'Path to GTFS .zip file.'
+    required: true
+  routes:
+    description: 'List of routes and types.'
+    required: true
+  output_file:
+    description: 'Path to output .zip file.'
+    required: false
+    default: ''
+
+runs:
+  using: 'docker'
+  image: 'Dockerfile'
diff --git a/modify_gtfs.py b/modify_gtfs.py
new file mode 100644
index 0000000..5edc79f
--- /dev/null
+++ b/modify_gtfs.py
@@ -0,0 +1,42 @@
+import os
+import sys
+import zipfile
+import pandas as pd
+import tempfile
+
+
+def modify_routes(gtfs_file, routes, output_file=None):
+    if not output_file:
+        output_file = gtfs_file
+
+    # unzip gtfs feed
+    with tempfile.TemporaryDirectory() as tmpdirname:
+        with zipfile.ZipFile(gtfs_file, 'r') as zip_ref:
+            zip_ref.extractall(tmpdirname)
+
+        # load routes.txt in a DataFrame
+        routes_path = os.path.join(tmpdirname, 'routes.txt')
+        routes_df = pd.read_csv(routes_path)
+
+        # edit route types
+        for route_change in routes.split(','):
+            route_id, new_type = route_change.split(':')
+            routes_df.loc[routes_df['route_id'] == route_id, 'route_type'] = int(new_type)
+
+        # save to routes.txt
+        routes_df.to_csv(routes_path, index=False)
+
+        # recreate zip file
+        with zipfile.ZipFile(output_file, 'w') as zip_ref:
+            for foldername, subfolders, filenames in os.walk(tmpdirname):
+                for filename in filenames:
+                    file_path = os.path.join(foldername, filename)
+                    zip_ref.write(file_path, os.path.relpath(file_path, tmpdirname))
+
+
+if __name__ == "main":
+    gtfs_file = sys.argv[1]
+    routes = sys.argv[2]
+    output_file = sys.argv[3]
+
+    modify_routes(gtfs_file, routes, output_file)
diff --git a/requirements.txt b/requirements.txt
new file mode 100644
index 0000000..a9109e6
--- /dev/null
+++ b/requirements.txt
@@ -0,0 +1 @@
+pandas~=2.2.2
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