I can't find DATABASE_URL in Vercel intergration

After I intergrated the Railway into Vercel, I can see lot of environment variables shared over there. But for some reason I can not see the DATABASE_URL, can someone help me to understand this? In Railway, I have 2 services, Redis and Postgres, ALL Redis' env vars show in Vercel, e.g. redis port, host, user etc. The Postgres, on the other hand, show nothing. I can't find any setting between them. Thanks in advance.