Step 1

• In the Firebase console, navigate to your project's settings by clicking the gear icon next to "Project Overview."

• Select Project settings, then go to the Service accounts tab

• Click the Generate new private key button. A confirmation pop-up will appear

• Click Generate Key to download a JSON file containing your service account credentials

• Store this JSON file securely and do not commit it to a public code repository, as it provides administrative access to your Firebase project

Step 2

• Open your project in the Railway dashboard and navigate to the service where you want to add the key, the one hosting the python code.

• Go to the Variables tab.

• Click on New Variable. It is recommended to add the key via the Raw Editor to ensure correct formatting.

• Create a variable named FIREBASE_CREDENTIALS_JSON.

• Open the JSON file you downloaded from Firebase, copy its entire content, and paste it as the value for the FIREBASE_CREDENTIALS_JSON variable

Step 3

Use it! You might need to adjust the dependencies you import according to your use case

import os
import json

# Retrieve the JSON credentials from the environment variable
firebase_creds_json_str = os.environ.get('FIREBASE_CREDENTIALS_JSON')

This is a very detailed reply and it will help you. I do not wan to take the bounty but here is the documentation for environment variables https://docs.railway.com/guides/variables#using-variables-in-your-services

Here is the documentation for how to get it from firebase https://firebase.google.com/docs/admin/setup#initialize_the_sdk_in_non-google_environments